compiler: fix type names for generic named structs

Without this change, the compiler would probably have worked just fine
but the generated types would look odd.

You can see in the test case that it now doesn't use `main.Point` but
rather the correct `main.Poin[float32]` etc.

compiler/testdata/generics.go

@@ -18,4 +18,7 @@ func Add[T Coord](a, b Point[T]) Point[T] {
 func main() {
 	var af, bf Point[float32]
 	Add(af, bf)
+
+	var ai, bi Point[int]
+	Add(ai, bi)
 }