c++: Fix bogus "does not declare anything" warning (PR 66159)

c++: Fix bogus "does not declare anything" warning (PR 66159)

G++ gives a bogus warning for 'struct A; using B = struct ::A;'
complaining that the elaborated-type-specifier doesn't declare anything.
That's true, but it's not trying to declare struct ::A, just refer to it
unambiguously. Do not emit the warning unless we're actually parsing a
declaration.

gcc/cp/ChangeLog:

	PR c++/66159
	* parser.c (cp_parser_elaborated_type_specifier): Do not warn
	unless in a declaration.

gcc/testsuite/ChangeLog:

	PR c++/66159
	* g++.dg/warn/forward-inner.C: Check alias-declaration using
	elaborated-type-specifier.