It's pointless to call *_rvalue_constant_expression when we're not using
the result. Also apply some drive-by cleanups.
gcc/cp/ChangeLog:
* parser.cc (cp_parser_constant_expression): Allow non_constant_p to be
nullptr even when allow_non_constant_p is true. Don't call
_rvalue_constant_expression when not necessary. Move local variable
declarations closer to their first use.
(cp_parser_static_assert): Don't pass a dummy down to
cp_parser_constant_expression.