compiler: Report unnamed receiver types.

gccgo used to crash when presented with an invalid receiver type in a
    method.  Instead, unnamed receiver types should report an error.
    
    Fixes golang/go#11557.
    
    Reviewed-on: https://go-review.googlesource.com/13245

From-SVN: r226899

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-fc9da313b4f5c13b4ac3bdddd98e699fd1c89613
+448d30b696461a39870d1b8beb1195e411300bfd
 
 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

@@ -1927,7 +1927,11 @@ Gogo::declare_function(const std::string& name, Function_type* type,
 	  return ftype->add_method_declaration(name, NULL, type, location);
 	}
       else
-	go_unreachable();
+        {
+          error_at(type->receiver()->location(),
+                   "invalid receiver type (receiver must be a named type)");
+          return Named_object::make_erroneous_name(name);
+        }
     }
 }