c++: Fix infinite looping with invalid operator [PR96137]

c++: Fix infinite looping with invalid operator [PR96137]

My r11-86 adjusted cp_parser_class_name to do

-  scope = parser->scope;
+  scope = parser->scope ? parser->scope : parser->context->object_type;
   if (scope == error_mark_node)
     return error_mark_node;

but that caused endless looping in cp_parser_type_specifier_seq (the
while (true) loop) in this invalid test, because we never set a parser
error, therefore cp_parser_type_specifier returned error_mark_node
instead of NULL_TREE, and we never issued the "expected type-specifier"
error.

At first I thought I'd just add cp_parser_simulate_error right before
the return, but that regresses crash81.C -- we'd emit multiple errors
for "T::X".  So the next best thing seemed to revert to pre-r11-86
behavior: return early when parser->scope is bad, otherwise proceed to
get the parser error.

gcc/cp/ChangeLog:

	PR c++/96137
	* parser.c (cp_parser_class_name): If parser->scope is
	error_mark_node, return it, otherwise continue.

gcc/testsuite/ChangeLog:

	PR c++/96137
	* g++.dg/parse/error63.C: New test.