d: Merge upstream dmd f5638c7b8.

d: Merge upstream dmd f5638c7b8.

Adds a CHECKENABLE enum, uses it for all contract parameters for
consistency in state checking.

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd f5638c7b8.
	* d-builtins.cc (d_init_versions): Use new CHECKENABLE enum.
	* d-codegen.cc (array_bounds_check): Likewise.
	(build_frame_type): Likewise.
	(get_frameinfo): Likewise.
	* d-lang.cc (d_init_options): Likewise.
	(d_init_options_struct): Don't initialize x_flag_bounds_check.
	(d_handle_option): Use new CHECKENABLE enum.
	(d_post_options): Likewise.  Set flag_bounds_check here.
	* expr.cc (ExprVisitor::visit(AssertExp *)): Use new CHECKENABLE enum.