ada: Crash on selected component of formal derived type in generic instance

ada: Crash on selected component of formal derived type in generic instance

The compiler crashes on an instantiation of a generic child unit G1.GC
that has a formal private extension P_Ext of a private type P declared
in the parent G1 whose full type has a component C, when analyzing a
selected component ACC.C whose prefix is of an access type coming from
an instantiation of another generic G2 where the designated type is
the formal type P_Ext (coming in from a formal type of G2).

gcc/ada/

	* sem_ch4.adb (Try_Selected_Component_In_Instance): Reverse if_statement
	clauses so that the testing for the special case of extensions of private
	types in instance bodies is done first, followed by the testing for the case
	of a parent type that's a generic actual type. In the extension case, apply
	Base_Type to the type actual in the test of Used_As_Generic_Actual, and add
	a test of Present (Parent_Subtype (Typ)).