Fix "PR c++/91073 if constexpr no longer works directly with Concepts."

Fix "PR c++/91073 if constexpr no longer works directly with Concepts."

This is a rather serious regression, filed in July 2019. Luckily the
fix is simple: is localized to parser.c and cp-tree.h in cp and boils
down to only a few lines.

Testing OK on x86_64-linux. Approved off-line by Jason Merrill.

	/cp
	PR c++/91073
	* cp-tree.h (is_constrained_auto): New.
	* parser.c (cp_parser_maybe_commit_to_declaration): Correctly
	handle concept-check expressions; take a cp_decl_specifier_seq*
	instead of a bool.
	(cp_parser_condition): Update call.
	(cp_parser_simple_declaration): Likewise.
	(cp_parser_placeholder_type_specifier): Correctly handle
	concept-check expressions.

	/testsuite
	PR c++/91073
	* g++.dg/concepts/pr91073-1.C: New.
	* g++.dg/concepts/pr91073-2.C: Likewise.