compiler: support alias to pointer type as method receiver

Fixes golang/go#28252.
    
    Reviewed-on: https://go-review.googlesource.com/c/160077

From-SVN: r268397

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-5af8ee0693944c280b1f529450dbfd4ec1ee451d
+2206f40fc1e0e1e2ba3eacb7388dd26b72729bde
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -1944,6 +1944,10 @@ Gogo::start_function(const std::string& name, Function_type* type,
 	  go_assert(at_top_level);
 	  Type* rtype = type->receiver()->type();
 
+	  while (rtype->named_type() != NULL
+		 && rtype->named_type()->is_alias())
+	    rtype = rtype->named_type()->real_type()->forwarded();
+
 	  // We want to look through the pointer created by the
 	  // parser, without getting an error if the type is not yet
 	  // defined.