Implemented
[[*time.traits.is.clock*]](https://eel.is/c++draft/time.traits.is.clock)
from [P0355R7](https://wg21.link/p0355r7).

This patch implements the C++20 feature `is_clock` and `is_clock_v`
based on the documentation [on
cppreference](https://en.cppreference.com/w/cpp/chrono/is_clock.html)

Fixes #166049.

This was part of https://wg21.link/p0355r7, but apparently never
implemented.

---------

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>
Co-authored-by: Hristo Hristov <zingam@outlook.com>

Fixes: #80895

Completes:
- LWG3359 <chrono> leap second support should allow for negative leap
seconds
- P1361R2 Integration of chrono with text formatting

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones

Fixes: #100432
Fixes: #100014

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones
- P1361 Integration of chrono with text formatting
- LWG3359 <chrono> leap second support should allow for negative leap
seconds

While implementing this feature and its associated LWG issues it turns
out
- LWG3316 Correctly define epoch for utc_clock / utc_timepoint only
added non-normative wording to the standard.

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones
- P1361 Integration of chrono with text formatting
- LWG3359 <chrono> leap second support should allow for negative leap
seconds

This patch implements the forwarding to frozen C++03 headers as
discussed in
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc. In the
RFC, we initially proposed selecting the right headers from the Clang
driver, however consensus seemed to steer towards handling this in the
library itself. This patch implements that direction.

At a high level, the changes basically amount to making each public
header look like this:

```
// inside <vector>
#ifdef _LIBCPP_CXX03_LANG
#  include <__cxx03/vector>
#else
  // normal <vector> content
#endif
```

In most cases, public headers are simple umbrella headers so there isn't
much code in the #else branch. In other cases, the #else branch contains
the actual implementation of the header.

Including The frozen C++03 headers results in a lot of formatting
changes in the main headers, so this splits these changes into a
separate commit instead.

This is part of
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc.