c++: Fix checking assert upon invalid class definition [PR116740]

c++: Fix checking assert upon invalid class definition [PR116740]

A checking assert triggers upon the following invalid code since
GCC 11:

=== cut here ===
class { a (struct b;
} struct b
=== cut here ===

The problem is that during error recovery, we call
set_identifier_type_value_with_scope for B in the global namespace, and
the checking assert added via r11-7228-g8f93e1b892850b fails.

This patch relaxes that assert to not fail if we've seen a parser error
(it a generalization of another fix done to that checking assert via
r11-7266-g24bf79f1798ad1).

	PR c++/116740

gcc/cp/ChangeLog:

	* name-lookup.cc (set_identifier_type_value_with_scope): Don't
	fail assert with ill-formed input.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/crash80.C: New test.