c++: more dummy non_constant_p arg avoidance

c++: more dummy non_constant_p arg avoidance

As a follow-up to Marek's r14-3088-ga263152643bbec, this patch makes
us avoid passing an effectively dummy non_constant_p argument in two
more spots in the parser so that we further avoid unnecessary
constantness checks from cp_parser_constant_expression.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_parenthesized_expression_list_elt): Pass
	nullptr as non_constant_p to cp_parser_braced_list if our
	non_constant_p is null.
	(cp_parser_initializer_list): Likewise to
	cp_parser_initializer_clause.  Avoid inspecting
	clause_non_constant_p if it's uninitialized.