cl: remove Underlying() call to reject string type aliases in rewrites

Type aliases like `type T string` are no longer supported for
-ldflags -X rewrites. Only direct *string types are now allowed.

- Removed Underlying() call from isStringPtrType
- Added TestRewriteIgnoresStringAlias test case

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <8459+cpunion@users.noreply.github.com>

cl/compile.go

@@ -151,7 +151,7 @@ func (p *context) isStringPtrType(typ types.Type) bool {
 	if !ok {
 		return false
 	}
-	basic, ok := ptr.Elem().Underlying().(*types.Basic)
+	basic, ok := ptr.Elem().(*types.Basic)
 	return ok && basic.Kind() == types.String
 }