cmd/cgo: reject attempts to declare methods on C types
This change causes cgo to emit an error (with the same
message as the compiler) when it encounters a declaration
of a method whose receiver type is C.T or *C.T.
Conceptually, C is another package, but unfortunately
the desugaring of C.T is a type within the same package,
causing the previous behavior to accept invalid input.
It is likely that at least some users are intentionally
exploiting this behavior, so this may break their build.
We should mention it in the release notes.