This is our emscripten-specific configuration file and was mistakenly
deleted when I ran update_libcxx.py.

This file was added as a part of LLVM 18 update (emscripten-core#21638) in
emscripten-core@8d51927
and mistakenly deleted when I ran update_libcxx.py. This file was copied
from
https://github.com/llvm/llvm-project/blob/aadaa00de76ed0c4987b97450dd638f63a385bed/libcxx/vendor/llvm/default_assertion_handler.in, so this also updates the file with the newest `default_assertion_handler.in`.

These Emscripten-specific files were mistakenly deleted when I ran
update_libcxx.py.

This changed in llvm/llvm-project#88968.

https://github.com/llvm/llvm-project/blob/aadaa00de76ed0c4987b97450dd638f63a385bed/libcxx/src/expected.cpp
was added in llvm/llvm-project#87390 and this
file assumes C++23 to be compiled. Apparently libc++ sources are always
built with C++23 so they don't guard things against it in `src/`:
llvm/llvm-project#87390 (comment)

This also bumps libc++abi to C++23 because... why not

We have excluded files in
https://github.com/emscripten-core/emscripten/tree/main/system/lib/libcxx/src/support/win32.
This is a new file added in this directory in
llvm/llvm-project#83575.

We disabled C++20 time zone support in LLVM 18 update (emscripten-core#21638):
emscripten-core@df9af64

The list of source files related to time zone support has changed in
llvm/llvm-project#74928, so this commit reflects
it.

Only `char`, `wchar`, `char8`, `char16`, and `char32` are valid
specialization for `std::basic_string`:
https://en.cppreference.com/w/cpp/string/basic_string

But libc++ had a base template for `basic_string` that allows any type
to be passed for a long time. It looks there have been several attempts
to remove this but they restored it afterwards due to some complaints,
in chronological order:
llvm/llvm-project@aeecef0
llvm/llvm-project@08a0faf
llvm/llvm-project@e30a148
llvm/llvm-project#66153
llvm/llvm-project#72694

The last one, llvm/llvm-project#72694,
eventually removed it. So `std::basic_string<unsigned_char>` is not
allowed anymore.

Not sure why but some of them decreasd by ~3%. Increases don't seem to
be meaningful; they are usually ~0.3%.

Only `char`, `wchar`, `char8`, `char16`, and `char32` are valid
specialization for `std::basic_string`:
https://en.cppreference.com/w/cpp/string/basic_string

But libc++ had a base template for `basic_string` that allowed any type
to be passed for a long time. It looks there have been several attempts
to remove this after which they restored it due to complaints, in
chronological order:
llvm/llvm-project@aeecef0
llvm/llvm-project@08a0faf
llvm/llvm-project@e30a148
llvm/llvm-project#66153
llvm/llvm-project#72694

The last one, llvm/llvm-project#72694,
eventually removed it. So `std::basic_string<unsigned_char>` is not
allowed anymore. This removes all uses of
`std::basic_string<unsigned_char>` from embind.

This needs to be done to update libc++ to LLVM 19 (emscripten-core#22994). I'm
uploading this as a separate PR because this removes a functionality
from embind.

…ibcxxabi_19