c++: fix template parm count leak

c++: fix template parm count leak

I noticed that we could leak parser->num_template_parameter_lists with
erroneous specializations.  We'd increment, notice a problem and then
bail out.  This refactors cp_parser_explicit_specialization to avoid
that code path.  A couple of tests get different diagnostics because
of the fix.  pr39425 then goes to unbounded template instantiation
and exceeds the implementation limit.

	gcc/cp/
	* parser.c (cp_parser_explicit_specialization): Refactor
	to avoid leak of num_template_parameter_lists value.
	gcc/testsuite/
	* g++.dg/template/pr39425.C: Adjust errors, (unbounded
	template recursion).
	* g++.old-deja/g++.pt/spec20.C: Remove fallout diagnostics.