Objective-C++ : Fix ICE in potential_constant_expression_1.

Objective-C++ : Fix ICE in potential_constant_expression_1.

We cannot, as things stand, handle Objective-C tree codes in
the switch and deal with this by calling out to a function that
has a dummy version when Objective-C is not enabled.

Because of the way the logic works (with a fall through to a
'sorry' in case of unhandled expressions), the function reports
cases that are known to be unsuitable for constant exprs. The
dummy function always reports 'false' and thus will fall through
to the 'sorry'.

gcc/c-family/ChangeLog:

	* c-objc.h (objc_non_constant_expr_p): New.
	* stub-objc.c (objc_non_constant_expr_p): New.

gcc/cp/ChangeLog:

	* constexpr.c (potential_constant_expression_1): Handle
	expressions known to be non-constant for Objective-C.

gcc/objc/ChangeLog:

	* objc-act.c (objc_non_constant_expr_p): New.

(cherry picked from commit 878cffbd9e6e1b138a6e0d362e7b29bc0a259940)