Use ErrnoCheckingTest harness to clear out / validate errno value
before/after the test respectively. Clean up explicit libc_errno.h
inclusions which is unnecessary, since no test modifies errno directly.

While working on strftime I noticed some constants were being defined in
unexpected places. One thing led to another, and I ended up doing a
major cleanup of the time functions.

What's included:
All uses of <time.h> in /src and /test removed (except for LibcTest.cpp)
The various time constants have been moved to time_constants.h, and the
`time_constants` namespace.
struct tm gets its own type indirection header now.

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.

This patch applies the lint rules described in the previous patch. There
was also a significant amount of effort put into manually fixing things,
since all of the templated functions, or structs defined in /spec, were
not updated and had to be handled manually.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D114302

gmtime and gmtime_r share the same common code. They call gmtime_internal
a static inline function. Thus added only validation tests for gmtime_r.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D99046