go/ssa: canonicalize receiver types to avoid creating duplicate thunk functions

+ Tests:
  Extend function name uniqueness check to exported wrappers.
  Check that expected synthetic functions are created, reachable, and not duplicated.

Change-Id: I0e87ebb2712e33e1f49da3fa9a9dde0085bf3850
Reviewed-on: https://go-review.googlesource.com/2013
Reviewed-by: Robert Griesemer <gri@golang.org>

go/ssa/builder_test.go

@@ -406,8 +406,7 @@ var (
 		delete(want, name)
 
 		if wantDescr != fn.Synthetic {
-			t.Errorf("(%s).Synthetic = %q, want %q",
+			t.Errorf("(%s).Synthetic = %q, want %q", name, fn.Synthetic, wantDescr)
-				name, fn.Synthetic, wantDescr)
 		}
 	}
 	for fn, descr := range want {