libc on macOS does not provide at_quick_exit or quick_exit. This allows
modules to build on macOS and defer any errors to usage site of these
symbols.
Fixes: https://github.com/llvm/llvm-project/issues/77559
This adds the std.compat module. The patch contains a bit of refactoring
to avoid code duplication between the std and std.compat module.
Implements parts of
- P2465R3 Standard Library Modules std and std.compat