The string overload will be deprecated soon, similar to other functions
in TargetRegistry.

lookupTarget takes StringRef and internally creates an instance of
std::string with the StringRef as part of constructing Triple, so we
don't need to create temporary instances of std::string on our own.

Adds a def file to have a single location where tested language versions
are specified. Removes the need to update multiple locations in the
testing infrastructure to add a new language version to be tested. Test
instatiation can now include all languages without needing to specify
them.
This patch also adds pretty printing for instantiated test names. That
means, that a test instantiated with C++23 will have the name
`...TestSuite/TestName/CXX23` instead ending with some number (index of
the argument for instantiation of the test), which provides a better
experience when encountering a test failure with a specific language
version. The suffix will also contain an `_win` if the target contains
`win`.

---------

Co-authored-by: Sirraide <aeternalmail@gmail.com>

Note that this patch will be necessary to fix
`forEachArgumentWithParam()` and `forEachArgumentWithParamType()`
matchers for deducing "this"; which is my true motivation. There the bug
is that with explicit obj params, one should not adjust the number of
arguments in presence of `CXXMethodDecls`, and this causes a mismatch
there mapping the argument to the wrong param. But, I'll come back there
once we have this matcher.

There is a discrepancy between how clangd processes CDB loaded from
JSON file on disk and pushed via LSP. Thus the same CDB pushed via
LSP protocol may not work as expected. Some difference between these two
paths is expected but we still need to insert driver mode and target from
binary name and expand response files.

Test Plan: check-clang-tools

Differential Revision: https://reviews.llvm.org/D143436

Differential Revision: https://reviews.llvm.org/D121176

Tests that need ASTs have to deal with the awkward control flow of
FrontendAction in some way. There are a few idioms used:
 - don't bother with unit tests, use clang -dump-ast
 - create an ASTConsumer by hand, which is bulky
 - use ASTMatchFinder - works pretty well if matchers are actually
   needed, very strange if they are not
 - use ASTUnit - this yields nice straight-line code, but ASTUnit is a
   terrifically complicated library not designed for this purpose

TestAST provides a very simple way to write straight-line tests: specify
the code/flags and it provides an AST that is kept alive until the
object is destroyed.
It's loosely modeled after TestTU in clangd, which we've successfully
used for a variety of tests.

I've updated a couple of clang tests to use this helper, IMO they're clearer.

Differential Revision: https://reviews.llvm.org/D123668

Add llvm_unreachable after switch statement for TestLanguage enum

Summary:
The unittest for AST matchers has its own way to specify language
standards. I unified it with the shared infrastructure from
libClangTesting.

Reviewers: jdoerfert, hlopko

Reviewed By: hlopko

Subscribers: mgorny, sstefan1, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81150