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

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This PR address issue #89002.

#### Changes in this PR

* Added a simple implementation of `cpp::lock_guard` (an equivalent of
`std::lock_guard`) in libc/src/__support/CPP inspired by the libstdc++
implementation
* Added tests for `cpp::lock_guard` in
/libc/test/src/__support/CPP/mutex_test.cpp
* Replaced all references to `MutexLock` with `cpp::lock_guard`

---------

Co-authored-by: Guillaume Chatelet <gchatelet@google.com>

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

Reviewed By: michaelrj, thesamesam

Differential Revision: https://reviews.llvm.org/D153729

The libc's equivalent of std::expected has a non-standard and
non-explicit operator T - https://github.com/llvm/llvm-project/issues/62738

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D152270

This patch updates the struct dirent to be on par with glibc (by adding
a missing d_type member) and update the readdir call to use SYS_getdents64
instead of SYS_getdents.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D147738

Fixes https://github.com/llvm/llvm-project/issues/61071

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D145191