ada: Errors on instance of Multiway_Trees with discriminated type

ada: Errors on instance of Multiway_Trees with discriminated type

The compiler may report various type conflicts on an instantiation
of the generic package Ada.Containers.Multiway_Trees with an actual
for Element_Type that is a nonprivate actual type with discriminants
that has a discriminant-dependent component of a private type (such
as a Bounded_Vector type). The type errors occur on an aggregate
of the implementation type Tree_Node_Type within the body of
Multiway_Trees, where the aggregate has a box-defaulted association
for the Element component. (Such type errors could of course arise
in other cases of generic instantiations that follow a similar type
model.)

In the case where the discriminant-dependent component type has a
default-initialization procedure (init proc), the compiler was handling
box associations for such components by expanding the topmost box
association into subaggregates that themselves have box associations,
and didn't properly account for discriminant-dependent subcomponents of
private types. This could be fixed internally in Propagate_Discriminants,
but it seems that the entire machinery for dealing with such subcomponent
associations is unnecessary, and the topmost component association can
be handled directly as a default-initialized box association.

gcc/ada/

	* sem_aggr.adb (Add_Discriminant_Values): Remove this procedure.
	(Propagate_Discriminants): Remove this procedure.
	(Resolve_Record_Aggregate): Remove code (the Capture_Discriminants
	block statement) related to propagating discriminants and
	generating initializations for subcomponents of a
	discriminant-dependent box-defaulted subcomponent of a nonprivate
	record type with discriminants, and handle all top-level
	components that have a non-null base init proc directly, by
	calling Add_Association with "Is_Box_Present => True". Also,
	combine that elsif clause with the immediately preceding elsif
	clause, since they now both contain the same statement (calls to
	Add_Association with the same actuals).