This patch removes all of the frozen headers which don't provide
anything. Basically any header that's C++11-or-later is removed from the
frozen headers.

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

This is a continuation of what's been started in #89178.

As a drive-by, this also changes the PSTL macro to say `EXPERIMENTAL`
instead of `INCOMPLETE`.

Some templates in the standard library are illegal to specialize for users
(even if the specialization contains user-defined types). The [[clang::no_specializations]]
attribute allows marking such base templates so that the compiler will
diagnose if users try adding a specialization.

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.

GCC 13 introduced the type trait `__remove_reference`. We can simplify
the implementation of `remove_reference` a bit by using it.

Originally, we used __libcpp_verbose_abort to handle assertion failures.
That function was declared from all public headers. Since we don't use
that mechanism anymore, we don't need to declare __libcpp_verbose_abort
from all public headers, and we can clean up a lot of unnecessary
includes.

This patch also moves the definition of the various assertion categories
to the <__assert> header, since we now rely on regular IWYU for these
assertion macros.

rdar://105510916

This allows backends to customize arbitrary parallel algorithms, which was requested pretty often.

Reviewed By: #libc, ldionne

Spies: arichardson, miyuki, crtrott, dalg24, __simt__, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D149686

This removes the need for a custom libc++ build to have a basic set of PSTL algorithms.

Reviewed By: ldionne, #libc

Spies: miyuki, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D149624