Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This replaces the old macros LIBC_COPT_TEST_USE_FUCHSIA and
LIBC_COPT_TEST_USE_PIGWEED with LIBC_COPT_TEST_ZXTEST and
LIBC_COPT_TEST_GTEST, respectively.  These are really not about
whether the code is in the Fuchsia build or in the Pigweed build,
but just about what test framework is being used.  The gtest
framework can be used in many contexts, and the zxtest framework
is not always what's used in the Fuchsia build.

The test/UnitTest/Test.h wrapper header now provides the macro
LIBC_TEST_HAS_MATCHERS() for use in `#if` conditionals on use of
gmock-style matchers, replacing `#if` conditionals that test the
framework selection macros directly.

Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:

    warning: header guard does not follow preferred style [llvm-header-guard]

This is because many of our header guards start with `__LLVM` rather than
`LLVM`.

To filter just these warnings:

    $ ninja -k2000 libc-lint 2>&1 | grep llvm-header-guard

To automatically apply fixits:

    $ find libc/src libc/include libc/test -name \*.h | \
        xargs -n1 -I {} clang-tidy {} -p build/compile_commands.json \
        -checks='-*,llvm-header-guard' --fix --quiet

Some manual cleanup is still necessary as headers that were missing header
guards outright will have them inserted before the license block (we prefer
them after).

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

This is a follow up to https://reviews.llvm.org/D152503

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D152533