c++: concept in default argument [PR109859]

1) We're hitting the assert in cp_parser_placeholder_type_specifier.
It says that if it turns out to be false, we should do error() instead.
Do so, then.

2) lambda-targ8.C should compile fine, though.  The problem was that
local_variables_forbidden_p wasn't cleared when we're about to parse
the optional template-parameter-list for a lambda in a default argument.

	PR c++/109859

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_lambda_declarator_opt): Temporarily clear
	local_variables_forbidden_p.
	(cp_parser_placeholder_type_specifier): Turn an assert into an
	error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-defarg3.C: New test.
	* g++.dg/cpp2a/lambda-targ8.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>