[Ada] Error on return of object whose full view has undefaulted discriminants

[Ada] Error on return of object whose full view has undefaulted discriminants

The compiler wrongly reports an error about the expected type not
matching the same-named found type in a return statement for a function
whose result type has unknown discriminants when the full type is tagged
and has an undefaulted discriminant, and the return expression is an object
initialized by a function call. The processing for return statements that
creates an actual subtype based on the return expression type's underlying
type when that type has discriminants, and converts the expression to
the actual subtype, should only be done when the underlying discriminated
type is mutable (i.e., has defaulted discriminants). Otherwise the
unchecked conversion to the actual subtype (of the underlying full type)
can lead to a resolution problem later within Expand_Simple_Function_Return
in the expansion of tag assignments (because the target type of the
conversion is a full view and does not match the partial view of
the function's result type).

gcc/ada/

	* exp_ch6.adb (Expand_Simple_Function_Return) Bypass creation of an actual
	subtype and unchecked conversion to that subtype when the underlying type
	of the expression has discriminants without defaults.