Here the 'decltype' is missing '(auto)', so open_paren was NULL, and trying
to get its location is a SEGV.  Using matching_parens avoids that problem.

gcc/cp/ChangeLog
2020-04-07  Jason Merrill  <jason@redhat.com>

	PR c++/94481
	* parser.c (cp_parser_placeholder_type_specifier): Use
	matching_parens.