This pr is to resovle #126948

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Co-authored-by: Joseph Huber <huberjn@outlook.com>

In the process of adding strftime (#122556) I wrote this utility class
to simplify reading from a struct tm. It provides helper functions that
return basically everything needed by strftime. It's not tested
directly, but it is thoroughly exercised by the strftime tests.

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.

The 32-bit time_t rolls over to a value with its high bit set at
2038-01-19 03:14:07. The overflow check for this in mktime() was
checking each individual component of the time: reject if year>2038, or
if month>1, or if day>19, etc. As a result it would reject valid times
before the overflow point, because a low-order component was out of
range even though a higher-order one makes the time as a whole safe. The
earliest failing value is 2145916808 == 2038-01-01 00:00:08, in which
only the seconds field 'overflows'.

Fixed so that if any component is _less_ than its threshold value, we
don't check the lower-order components.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

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