No longer hang on typeof of a function type

We were calling `isFunctionProtoType()` on a `ParsedType` rather than
creating a valid semantic type first and calling the function on that.
The call to `isFunctionProtoType()` would eventually call
`getUnqualifiedDesugaredType()`, which loops indefinitely until we get
a desugared type and a `ParsedType` will never finish desugaring.

Fixes https://github.com/llvm/llvm-project/issues/64713