This adds a test that consists of compiling `#include <...>`,
pretty much alone, for each public header file in each different
language mode (`-std=...` compiler switch) with -Werror and many
warnings enabled.

There are several headers that have bugs when used alone, and
many more headers that have bugs in certain language modes.  So
for now, compiling the new tests is gated on the cmake switch
-DLLVM_LIBC_BUILD_HEADER_TESTS=ON.  When all the bugs are fixed,
the switch will be removed so future regressions don't land.

I don't quite recall why I added those in the first place. These tests build
without diagnostics for both clang and GCC with this fix.

Fixes: #114653

These are failing to link for some buildbots. It's not immediately clear
why,
disable these and add a todo to investigate.

Link: #111403
Link: #114566
Link: #114618

This avoids -Wmacro-redefinition diagnostics observed when building the
libc_include_tests ninja target.

For example, the signbit_test will attempt to include BOTH our math-macros.h
(via math-function-macros.h), and the system's math.h (via hdr/math_macros.h).

While it's nice that we can get some coverage of the headers we will provide to
end users of fullbuilds in CI of overlay builds, it's not worth chasing each
individual conflict and disabling some include tests as conflicts arise.

Disable the include tests unless `-DLLVM_LIBC_FULL_BUILD=ON` is specified.

Refer: 7.3.1 from [ISO
SPEC](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf)

I have added complex variants of F16 and F128 in libc doc but have
omitted support for them since we will have to first investigate how
their support matrix for clang and gcc looks like, and then add header
guards for them accordingly. Planning to add them in follow up PRs once
this gets landed.

#109201

reapply #109615

Reverts llvm/llvm-project#109615

#109201