Summary:
This is a dependency on `strftime` which we provide, so we should have
this.

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 patch fixes several warnings thrown by clang about an uninitialized
member of struct tm, tm_isdst.

Weirdly, gcc doesn't complain about it, probably this member is never
read in the tests.

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

Previously, these tests expected that calling mktime with a struct tm
that caused overlow to succeed with return -1
(TimeConstants::OUT_OF_RANGE_RETURN_VALUE), however, the Succeeds call
expects the errno to be zero (no failure).

This patch fixes the expected calls to fail with EOVERFLOW. These tests
are only enabled to 32-bit systems, and are probably not being tested on
the arm32 buildbot, that's why this was not a problem before.

This patch changes a test case that tests for overflow when time_t is
32-bit long, however, it was checking size_t instead of time_t.

This in on par with other testcases that correctly check the size of
time_t (asctime_test.cpp, gmtime_r_test.cpp and gmtime_test.cpp).

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159113

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