ada: Fix inheritance of Is_Primitive and legality check for nonoverridable aspects

ada: Fix inheritance of Is_Primitive and legality check for nonoverridable aspects

This patch fixes the identification of inherited subprograms as primitive
operations via the Is_Primitive flag. This is essential in the context of the
new legality check which makes sure that, if any subprogram denoted by a
nonoverridable aspect of a type T with formal or return of either type T or
access T, then all denoted subprograms should be primitive operations. Note
that all valid interpretations of a subprogram are denoted by the aspect under
evaluation, all of these needs to be primitive then.

This is a respin of eng/toolchain/gnat!2039

gcc/ada/ChangeLog:

	* sem_ch13.adb (Check_Nonoverridable_Aspect_Subprograms): Add the new
	legality check in Check_Nonoverridable_Aspect_Subprograms for
	nonoverridable aspects to check whether the denoted subprograms satisfy
	MR 13.1.1(18.4/6), otherwise we emit an error. Fix spacing.
	* sem_ch6.adb (New_Overloaded_Entity): Set Is_Primitive flag
	for inherited primitives, and filter out homonym candidates without a
	function specification as parents.