[clang] [Clang][WebAssembly] Fix crash when using __funcref in C++ code (PR #176237)

[Paulo Matos via cfe-commits]

pmatos wrote:

> I assume enabling this mangler doesn't change the mangling of any symbols not in addrspace 0?

This doesn't change the mangling of anything except funcref which before was hitting an llvm_unreachable. externref mangling stays the same as well as everything else. This just ensures we can mangle function pointers to addrspace 20, so mangling will look like: `PU4AS20FvvE`.

https://github.com/llvm/llvm-project/pull/176237