ada: Error on instantiation with defaulted formal type referencing other formal type

The compiler wasn't accounting for default subtypes on generic formal types
that reference other formal types of the same generic, leading to errors
about invalid subtypes. Several other problems that could lead to blowups
or incorrect errors were noticed through testing related cases and fixed
along the way.

gcc/ada/ChangeLog:

	* sem_ch12.adb (Analyze_One_Association): In the case of a formal type
	that has a Default_Subtype_Mark that does not have its Entity field set,
	this means the default refers to another formal type of the same generic
	formal part, so locate the matching subtype in the Result_Renamings and
	set Match's Entity to that subtype prior to the call to Instantiate_Type.
	(Validate_Formal_TypeDefault.Reference_Formal): Add test of Entity being
	Present, to prevent blowups on End_Label ids (which don't have Entity set).
	(Validate_Formal_Type_Default.Validate_Derived_Type_Default): Apply
	Base_Type to Formal.
	(Validate_Formal_Type_Default): Guard interface-related semantic checks
	with a test of Is_Tagged_Type.