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

[Paulo Matos via cfe-commits]

pmatos wrote:

The current patch implements successfully all the refactoring needed for externref. I will work on funcref next.

This refactoring was triggered by the discussion at #71069 .
Also the issue #71388 is currently blocking this. I include a workaround in the current patch.

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