[clang] [llvm] [WebAssembly] Refactor Wasm Reference Types as TargetExtType (PR #71540)

[Paulo Matos via cfe-commits]

pmatos wrote:

> Very cool! LGTM so far. It's really nice that we can delete that entire pass out of the backend. Where can I read more about these new target extension types?

There's not a lot but there's something here:
https://llvm.org/docs/LangRef.html#target-extension-type

and of course, the api:
https://llvm.org/doxygen/classllvm_1_1TargetExtType.html

https://github.com/llvm/llvm-project/pull/71540