[libcxx] [libcxxabi] don't throw+catch in std::make_exception_ptr when building with libcxxabi

[itrofimow]

Summary

Instead of throwing an exception allocate its memory and initialize it
explicitly. Makes std::make_exception_ptr more efficient since no stack
unwinding is needed.

[itrofimow]

• + clang-format

[itrofimow]

• cleanup

[itrofimow]

• + clang-format, fix libcxx.imp

[itrofimow]

Hi!

Inspired by what libstdc++ does: we don't have to throw + catch (thus unwinding) an exception when creating exception_ptr,
because libcxxabi knows exactly how to create an exception_ptr from the given object.

Could you please validate this idea and if this seems reasonable guide me on what and where should be adjusted to make
generated_files/modules/etc. tests pass?

[itrofimow]

• wip on making the build green

[itrofimow]

• wip on making the build green

[itrofimow]

• regenerate ignore_format.txt

[itrofimow]

• + clang-format
• cleanup
• + clang-format, fix libcxx.imp
• wip on making the build green
• wip on making the build green
• regenerate ignore_format.txt
• bits of formatting
• wip on making the build green

[itrofimow]

• wip on making the build green

[itrofimow]

• rebase to trigger build

[philnik]

Since we are moving to GitHub PRs and there hasn't been any activity here yet, could you move this patch over?