c++: Fix crash with template spec in different namespace [PR94255]

c++: Fix crash with template spec in different namespace [PR94255]

This is an ICE on invalid, because we're specializing S::foo in the
wrong namespace.  cp_parser_class_specifier_1 parses S::foo in M
and then it tries to push the nested-name-specifier of foo, which is
S.  By that, we're breaking the assumption of push_inner_scope that
the pushed scope must be a scope nested inside current scope: current
scope is M, but the namespace context of S is N, and N is not nested
in M, so we fell into an infinite loop in push_inner_scope_r.

(cp_parser_class_head called check_specialization_namespace which already
gave a permerror.)

	PR c++/94255
	* parser.c (cp_parser_class_specifier_1): Check that the scope is
	nested inside current scope before pushing it.

	* g++.dg/template/spec41.C: New test.