[clang] [llvm] [WebAssembly] Represent reference types as TargetExtType (PR #203165)

[Hood Chatham via cfe-commits]

hoodmane wrote:

Incidentally, I noticed while writing this that we never emit `call_ref`. We should ideally check whether the `gc` target feature is present and if so emit a `call_ref` to call a function pointer rather than using a dummy table. I'll try to do this in a separate PR.

https://github.com/llvm/llvm-project/pull/203165