[Ada] Fix warning about generic subprograms withed but not referenced

[Ada] Fix warning about generic subprograms withed but not referenced

For warnings about unreferenced entities and unused WITH clauses we
typically exclude references outside of the extended main source unit.
However, we include references to variables of formal private types to
warn in the instance if the corresponding type is not a fully
initialized type.

This special case was unnecessarily applied to references to other
entities of formal private types. In particular, we applied it to the
references to generic functions, where the full initialization of the
formal private type doesn't matter.

As an extreme example, this special case was unnecessarily applied to
"pragma Pure (Unchecked_Conversion)" in the runtime unit.

gcc/ada/

	* lib-xref.adb (Generate_Reference): Fix comment and implement
	it precisely.