ada: Overridden operation field not correctly set for controlling result wrappers

ada: Overridden operation field not correctly set for controlling result wrappers

Implicit wrapper overridings generated for functions with
controlling result when deriving with null extension may
have field Overridden_Operation incorrectly set, when making
several such derivations in succession. This happens because
overridings were assumed to come from source, and entities
generated by Derive_Subprograms were also assumed to be
derived from source subprograms. Overridden_Operation could
be set to the entity generated by Derive_Subprograms for the
same type, resulting in a cycle between Overriden_Operation
and Alias fields, causing non-termination in GNATprove.

gcc/ada/

	* sem_ch6.adb (Check_Overriding_Indicator) Remove Comes_From_Source filter.
	(New_Overloaded_Entity) Move up special case of LSP_Subprogram,
	and remove Comes_From_Source filter.