ada: Legal access discriminant default expression incorrectly rejected

ada: Legal access discriminant default expression incorrectly rejected

If a limited private partial view of a type has an access discriminant with
a default expression, and if the type (perhaps tagged, perhaps not) is
completed by deriving from an immutably limited type, then the default
discriminant expression should not be rejected.

gcc/ada/ChangeLog:

	* sem_ch6.adb (Check_Discriminant_Conformance): In testing whether
	a default expression is permitted for an access discriminant, we
	need to know whether the discriminated type is immutably limited.
	Handle another part of this test that cannot easily be handled in
	Sem_Aux.Is_Immutably_Limited. This involves declaring a new local
	function, Is_Derived_From_Immutably_Limited_Type.