[Ada] Fix hiding of user-defined operator that is not a homograph

[Ada] Fix hiding of user-defined operator that is not a homograph

This adds a missing test for the presence of a homograph when applying
the RM 8.4(10) clause about the visibility of operators, and removes
resolution code made obsolete by the change.  There is also a fixlet
for a previously undetected ambiguity in the runtime.

gcc/ada/

	* sem_res.adb (Resolve_Eqyality_Op): Remove obsolete code.
	(Resolve_Op_Not): Likewise.
	* sem_type.adb (Disambiguate): Add missing test for RM 8.4(10).
	* libgnat/s-dwalin.adb (Enable_Cache): Fix ambiguity.
	(Symbolic_Address): Likewise.

gcc/testsuite/

	* gnat.dg/equal7.adb: Add expected error messages (code is now
	illegal).