Hello, I merged this https://github.com/llvm/llvm-project/pull/114904 a
few mins ago and the tests failed because i did not add the header
`src/stdlib/_Exit.h` in `at_quick_exit_test.cpp` and `atexit_test.cpp`.

I ran both builds/tests and everything was good. thanks

This improves/fixes this pr
https://github.com/llvm/llvm-project/pull/114718. In this PR we removed
the _Exit proxy func because it was not needed. Instead we used
`LIBC_NAMESPACE::_Exit`

This finishes the work from
https://github.com/llvm/llvm-project/pull/114453 by adding proxy headers
for `malloc`, `realloc`, `free` and `aligned_alloc`.

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 part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.

Migrating all private STL code to the standard STL case but keeping it under the CPP namespace to avoid confusion.

Differential Revision: https://reviews.llvm.org/D130771

This is a reland of https://reviews.llvm.org/D130773

This reverts commit 7add0e5fdc5c7cb6f59f60cd436bf161cf9f9eb7.

Migrating all private STL code to the standard STL case but keeping it under the CPP namespace to avoid confusion.

Differential Revision: https://reviews.llvm.org/D130773