c++: tweak C++20 destructor template-id rule

While working on a larger change to destructor lookup I noticed that this
rule talks about declarators, but we weren't limiting the error to the case
where we're parsing a declarator.  I don't know if this actually broke
anything, since a CPP_TEMPLATE_ID would have to have been parsed once
before, but it's more correct this way.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_unqualified_id): Only complain about ~A<T> in
	a declarator.