ada: Missing legality check when type completed

An access discriminant is allowed to have a default value only if the
discriminated type is immutably limited. In the case of a discriminated
limited private type declaration, this rule needs to be checked when
the completion of the type is seen.

gcc/ada/

	* sem_ch6.adb (Check_Discriminant_Conformance): Perform check for
	illegal access discriminant default values when the completion of
	a limited private type is analyzed.
	* sem_aux.adb (Is_Immutably_Limited): If passed the
	not-yet-analyzed entity for the full view of a record type, test
	the Limited_Present flag
	(which is set by the parser).