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 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.

This removes transitive includes that are only in a header that is empty
in a given C++ version.

This is a relatively low cost way of reducing the include sizes in older
language modes compared to the effect. For example, in C++14 mode the
include time of `<algorithm>` is reduced from 198ms to 127ms.

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 reduces the time to include `<compare>` from 84ms to 36ms.

These changes make it possible to use __synth_three_way in modules. The
change from a lambda to a function is a Clang issue.

The change is list was needed since the compiler couldn't deduce the
comparison template argument.

Adds a few missing includes too.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D146545

Reviewed By: Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D140480