Reverts llvm/llvm-project#160413

Relates to https://github.com/llvm/llvm-project/issues/119281

Note:

1) As this PR enables `-Werror` for `libc` tests, it's very likely some
downstream CI's may fail / start failing, so it's very likely this PR
may need to be reverted and re-applied.

P.S.

I do not have merge permissions, so I will need one of the reviews to
merge it for me. Thank you!

Remove direct libc_errno.h inclusion and manipulation of libc_errno in
various unit tests, instead relying on ErrnoCheckingTest machinery.

This is the final mechanical change of migrating libc unit tests to
ErrnoCheckingTest - after it, all the unit tests relying on
ASSERT_ERRNO_* macro will be using ErrnoCheckingTest.h

https://github.com/llvm/llvm-project/issues/149993

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Along the way, couple of additional things have been done:

1. Move `ErrnoSetterMatcher.h` to `test/UnitTest` as all other matchers live
   there now.
2. `ErrnoSetterMatcher` ignores matching `errno` on GPUs.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151129

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D145274