Add libc++abi Objective-C exception bridge for WebAssembly

[HendrikHuebner]

This PR is the counterpart to llvm/llvm-project#215562.

For native WebAssembly exceptions, Clang emits references to the personality function __gxx_wasm_personality_v0.
In order to support Objective-C exception semantics for WASM targets, we could either define our own personality function, or box Objective-C exceptions inside of C++ exceptions and rely on emscriptens libc++abi / exception runtime to handle those correctly for us.

To support Objective-C catch semantics, we need to supply our own type info objects which override the can_catch method of libc++abi's internal __shim_type_info.

For now, this PR is Apple compatible but does not implement the non-compatible mode.

I'll leave this as future work. I'm not sure whether AppleCompatibleMode is currently implemented as it was originally intended, since disabling compatibility would affect both @catch and C++ catch handler semantics as it is currently written, making them both use static-type matching instead of dynamic-type matching.

[davidchisnall]

This looks very similar to the code in objcxx_eh_mingw, which also wraps C++ exceptions. Is there a reason to have an entirely new parallel implementation for WAsm?

[HendrikHuebner]

Conceptually its doing the same thing as objcxx_eh_private.h by wrapping Objective-C exceptions in C++ exceptions.

The problem is that both rely on internal implementation details from different standard libraries.

For targetting emscripten we need our type infos to be subclasses of libc++abi's __shim_type_info, which is itself a subclass of std::type_info.

The existing implementation relies on gcc/libsupc++ implementation details instead, where the std::type_info already contains the __do_catch member and whatever else we need to override.

I don't think these implementations can be merged, sadly.

Edit: maybe some things could be reused, such as the implementation of __do_catch/can_catch?