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 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.

Summary:
Having a consistent prefix makes selecting all of the llvm libc tests
easier on any platform that is also using the gtest framework.
This also modifies the TEST and TEST_F macros to enforce this change
moving forward.

Reviewers: sivachandra

Subscribers:

Summary:
Made all header files consistent based of this documentation: https://llvm.org/docs/CodingStandards.html#file-headers.
And did the same for all source files top of file comments.

Reviewers: sivachandra, abrachet

Reviewed By: sivachandra, abrachet

Subscribers: MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D77533

Header files included wrongly using <...> are now included using the
internal path names as the new unittest framework allows us to do so.

Reviewers: phosek, abrachet

Differential Revision: https://reviews.llvm.org/D72743

A toplevel target, `check-libc` has also been added.

Reviewers: abrachet, phosek

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D72177