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

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.

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 patch provides specific test macros to deal with `errno`.
This will help abstract away the differences between unit test and integration/hermetic tests in #79319.
In one case we use `libc_errno` which is a struct, in the other case we deal directly with `errno`.

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 test creates a time_t variable and assigns 0xfffffffffe1d7b01 which
overflows the maximum time_t value for 64-bit time_t, then checks if the
syscall fails and errno was set.

In systems with sizeof(time_t) == 4, the value is narrowed down to
0xfe1d7b01 and doesn't overflow, causing the test to fail.

This patch then disables the test on systems with 32 bits long time_t.

Along the way, couple of additional things have been done:

1. Move `ErrnoSetterMatcher.h` to `test/UnitTest` as all other matchers live
   there now.
2. `ErrnoSetterMatcher` ignores matching `errno` on GPUs.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151129

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.