This is an implementation for template functions of localtime.

Update for this pull request: Implementation as been removed from this
pull request and will be added to a new one. This is because this pull
request is getting big. This pull request will only contain template
functions in order to implement localtime.

Update: The implementation is available in
https://github.com/zimirza/llvm-project/tree/localtime_implementation.

---------

Co-authored-by: Зишан Мирза <zmirza@tutanota.de>
Co-authored-by: Zishan Mirza <zmirza@posteo.de>

https://github.com/llvm/llvm-project/issues/149993

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 pr is to resovle #126948

---------

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

On some systems time_t is 32 bit, causing build errors (with -Werror)
in get_epoch which attempts to implicitly convert an int64_t to a
time_t.

Fixes:

error: implicit conversion loses integer precision: 'int64_t' (aka 'long
long') to 'time_t' (aka 'int') [-Werror,-Wshorten-64-to-32]
      332 |     return mktime_internal(timeptr);
          |     ~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~

get_epoch calls mktime_internal which isn't constexpr. For now, just
remove the constexpr from get_epoch.

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.

This is an implementation of `ctime` and includes `ctime_r`.

According to documentation, `ctime` and `ctime_r` are defined as the
following:

```c
char *ctime(const time_t *timep);
char *ctime_r(const time_t *restrict timep, char buf[restrict 26]);
```

closes #86567

The `<time.h>` functions `asctime`, `gmtime`, `mktime` and their `_r`
variants are purely computational functions which convert between well
defined time representations and/or strings. There's no reason these
shouldn't be available in bare-metal builds of the library as well as
hosted ones: even if the library has no way to find out the time in
POSIX format, it might still see POSIX-style `time_t` values in input
data (e.g. network protocols) and need to interpret them.

The only obstacle to this was that the `out_of_range()` helper function
set `errno` to `EOVERFLOW`, which fails in a bare-metal build because
the extra POSIX error values aren't defined, including `EOVERFLOW`. So
I've made that assignment conditional on `EOVERFLOW` being defined.

Fixes #85556.