Most of these tests do not actually have a shell requirement. The shell
requirement ended up in the test either from cargo culting (from what I
can tell) or because the test authors actually meant to mark Windows as
unsupported. This prevents enablement of lit's internal shell within
clang.

Towards #102699.

Reviewers: rnk, efriedma-quic, Sirraide, petrhosek, ilovepi

Reviewed By: ilovepi

Pull Request: https://github.com/llvm/llvm-project/pull/156905

This reverts commit 4c80193a58a5c24e2bbebe291feb406191c4e2ab.

This relands the commit. The issues have theoretically been fixed.

This reverts commit 5a586375aa3a128dadc9473cfa196bf8588c2a82.

This breaks two buildbots with failures in
implicit-module-header-maps.cpp. No idea why these failures are
occurring.

https://lab.llvm.org/buildbot/#/builders/64/builds/5166
https://lab.llvm.org/buildbot/#/builders/13/builds/8725

This patch removes %T from clang lit tests. %T has been deprecated for
about seven years and is not reccomended as it is not unique to each
test, which can lead to races. This patch is intended to remove usage in
tree with the end goal of removing support for %T within lit.

Similar to previous cleanup.

While changing mips* tests, change some -no-integrated-as to the
recommended -fno-integrated-as.

`Driver::ClangExecutable` is derived from:

* (-canonical-prefixes default): `realpath` on the executable path
* (-no-canonical-prefixes) argv[0] (consult PATH if argv[0] is a word)

`Dir` and `ResourceDir` are derived from `ClangExecutable`. Both
variables are used to derive certain include and library paths.

`InstalledDir` is a related concept used to derive certain other paths.
`InstalledDir` is weird in the -canonical-prefixes mode: Clang
calls `make_absolute` but does not follow symlinks
(FIXME from 9ade6a9a747c49383ac747bd8ffaa6a0beaef71c).
This causes some search and library paths to be mix-and-matched.

The "Do a PATH lookup, if there are no directory components." logic
makes things worse.
`InstalledDir` is different when you invoke it via `PATH`:
```
% which clang
/usr/bin/clang
% clang -v |& grep InstalledDir
InstalledDir: /usr/bin
% /usr/lib/llvm-16/bin/clang -v |& grep InstalledDir
InstalledDir: /usr/lib/llvm-16/bin
```

I believe `InstalledDir` was a partial solution to
`-no-canonical-prefixes` and should be eventually removed.

This patch removes `SetInstallDir` and relies on Driver::Driver to set
`InstalledDir` to `Dir`. The behavior for regular file `clang` or
`-no-canonical-prefixes` is unchanged.

If a user creates a symlink to the regular file `clang` and uses the
default `-canonical-prefixes`, they now consistently get search and
library paths relative to the regular file `clang`, not mix-and-match
paths.

If a user creates a symlink to the regular file `clang` and replaces
some directorys from the actual installation, they should change the
symlink to a wrapper that calls the underlying clang with
`-no-canonical-prefixes`.

This fixes uses of the MSYS2 clang64 environment compilers, if another
set of GCC based compilers are available further back in PATH (which may
be explicitly added, or inherited unintentionally from other software
installed).

(The issue in the clang64 environment can be worked around somewhat by
installing *-gcc-compat packages which present aliases named
<triple>-gcc within the clang64 environment as well.)

This fixes https://github.com/msys2/MINGW-packages/issues/11495 and
https://github.com/msys2/MINGW-packages/issues/19279.

The MinGW compiler driver first tries to deduce the root of
the toolchain installation (either clang itself or a separate
cross mingw gcc installation). On top of this root, a number
of include and lib paths are added (some added unconditionally,
some only if they exist):
- <base>/x86_64-w64-mingw32/include
- <base>/include
- <base>/include/x86_64-w64-windows-gnu
(Some more are also added for libstdc++ and/or libc++.)

The first one is the one commonly used for MinGW targets so
far. For LLVM runtimes installed with the
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR option, the latter two are
used though (this is currently not the default, not yet at least).

For cross compiling, if base is a separate dedicated directory,
this is fine, but when using the sysroots of a distro-installed
cross mingw toolchain, base is /usr - and having /usr/include
in the include path for cross compilation is a potential
source for problems; see
https://github.com/llvm/llvm-project/issues/59871.

If not cross compiling though, <base>/include needs to be included
too. E.g. in the case of msys2, most headers are in e.g.
/mingw64/include while the compiler is /mingw64/bin/clang.

When cross compiling, if the sysroot has been explicitly set
by the user, keep <base>/include too. (In the case of a distro
provided cross gcc toolchain in /usr, the sysroot needs to be set
to /usr and not /usr/x86_64-w64-mingw32 though, to be able to find
libgcc files under /usr/lib/gcc/x86_64-w64-mingw32. So with such a
toolchain, setting the sysroot explicitly does retain the problem.)

All in all - this avoids adding /usr/include and /usr/lib to the
include/lib paths when doing mingw cross compilation with a
distro-provided sysroot in /usr/x86_64-w64-mingw32.

Test that the include directory is omitted in the mingw-sysroot.cpp
tests, when cross compiling. That test is only ever executed on
non-Windows hosts, since it uses symlinks to set up fake environments
with colocated compilers and header/lib directories. There aren't
really any current corresponding tests for the same implicit behaviours
when actually running on Windows.

Differential Revision: https://reviews.llvm.org/D141206

Account for backslashes in paths in mingw.cpp.

Testing clang with the <triple>-clang form seems to require the
x86 target to be enabled, when the triple is an x86 triple. Just
skip that aspect of the test, since the "clang --target=<triple>"
form should give enough test coverage here.

There are three functions that try to detect the right implicit
sysroot and libgcc directory setup to use
- One which looks for mingw sysroots located in
  <clangbin>/../<sysrootname>
- One which looks for a mingw-targeting gcc executables in the PATH
- One which looks in the <gccroot>/lib/gcc directory to find the
  right one to use, and the right specific triple used for arch
  specific directories in the gcc/libstdc++ install

These have mostly tried to look for executables named
"<arch>-w64-mingw32-gcc" or "mingw32-gcc" or subdirectories
named "<arch>-w64-mingw32" or "mingw32".

In the case of findClangRelativeSysroot, it also has looked
for directories with the name of the actual triple. This
was added in deff7536278d355977171726124f83aa4bb95419,
with the intent of looking for a directory matching exactly
the user provided literal triple - however the triple here
is the normalized one, not the one provided by the user on
the command line.

Improve and unify this logic somewhat:
- Always first look for things based on the literal triple
  provided by the user.
- Secondly look for things based on the normalized triple
  (which usually ends up as e.g. x86_64-w64-windows-gnu),
  accessed via the Triple which is passed to the constructor
- Then look for the common triple form <arch>-w64-mingw32

The literal triple provided by the user is available via
Driver::getTargetTriple(), but computeTargetTriple() may
change e.g. the architecture of it, so we need to
reapply the effective architecture on the literal triple
spelling from Driver::getTargetTriple().

Do this consistently for all of findGcc, findClangRelativeSysroot
and findGccLibDir (while keeping the existing plain "mingw32"
cases in findGcc and findGccLibDir too).

Fedora 37 started shipping mingw sysroots targeting UCRT,
in addition to the traditional msvcrt.dll, and these use
triples in the form <arch>-w64-mingw32ucrt - see
https://fedoraproject.org/wiki/Changes/F37MingwUCRT.

Thus, in addition to the existing default tested triples,
try looking for triples in the form <arch>-w64-mingw32ucrt,
to automatically find the UCRT sysroots on Fedora 37.
By explicitly setting a specific target on the Clang command
line, the user can be more explicit with which flavour is
to be preferred.

This should fix the main issue in
https://github.com/llvm/llvm-project/issues/59001.

Differential Revision: https://reviews.llvm.org/D138692