fixes #105309

fixes https://github.com/llvm/llvm-project/issues/105190

---------

Co-authored-by: Hui Xie <huixie@Mac.broadband>
Co-authored-by: Hui Xie <huixie@Huis-MacBook-Pro.local>

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.

That's what we (try to) do consistently for all other umbrella headers.

As a drive-by, remove the <__assert> header which is not mandated
anymore.

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.

Around half of the tests are based on the tests Arthur O'Dwyer's
original implementation of std::flat_map, with modifications and
removals.

partially implement #105190