compiler: Report invalid receiver types in function definitions.

    
    Fixes golang/go#12324.
    
    Reviewed-on: https://go-review.googlesource.com/13988

From-SVN: r227427

gcc/go/gofrontend/MERGE

-3f8feb4f905535448833a14e4f5c83f682087749
+672ac2abc52d8bd70cb9fb03dd4a32fdde9c438f
 
 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

@@ -1818,7 +1818,11 @@ Gogo::start_function(const std::string& name, Function_type* type,
 								  function);
 	    }
 	  else
-	    go_unreachable();
+            {
+              error_at(type->receiver()->location(),
+                       "invalid receiver type (receiver must be a named type)");
+              ret = Named_object::make_function(name, NULL, function);
+            }
 	}
       this->package_->bindings()->add_method(ret);
     }