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 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

Switch use of errno in src/time and test/src/time to libc_errno.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D145192

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.

A bunch of cleanup to supress the new warnings is also done.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D130723

Apply the formatting rules that were applied to the libc/src directory
to the libc/test directory, as well as the files in libc/utils that are
included by the tests. This does not include automated enforcement.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D116127

[libc] Introduce asctime, asctime_r to LLVM libc

asctime and asctime_r share the same common code. They call asctime_internal
a static inline function.

asctime uses snprintf to return the string representation in a buffer.
It uses the following format (26 characters is the buffer size) as per
7.27.3.1 section in http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2478.pdf.
The buf parameter for asctime_r shall point to a buffer of at least 26 bytes.

snprintf(buf, 26, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",...)

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D99686