[Ada] Incorrect determination of whether an expression is predicate-static

[Ada] Incorrect determination of whether an expression is predicate-static

The expression given in a Static_Predicate aspect specification is
required to be predicate-static. The implementation of this compile-time
check was incorrect in some cases. There were problems in both
directions: expressions that are not predicate-static were incorrectly
treated as though they were and vice versa. This led to both accepting
invalid code and to rejecting valid code.

gcc/ada/

	* sem_ch13.adb (Is_Predicate_Static): Do not generate warnings
	about subexpressions of enclosing expressions. Generate warnings
	for predicates that are known to be always true or always false,
	except in the case where the predicate is expressed as a Boolean
	literal. Deal with non-predicate-static expressions that have
	been transformed into predicate-static expressions.  Add missing
	Is_Type_Ref call to N_Membership_Test case.