ada: Use-before-definition of a component of discriminated aggregate's itype.

ada: Use-before-definition of a component of discriminated aggregate's itype.

In some cases involving assigning an aggregate to a formal parameter of
an unconstrained discriminated subtype that has a Dynamic_Predicate, and where
the discriminated type also has a component of an unconstrained discriminated
subtype, the front end generates a malformed tree which causes a compilation
failure when the backend fails a consistency check.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Convert_To_Assignments): Add calls to Ensure_Defined
	before generating assignments to components that could be
	associated with a not-yet-defined itype.