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

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

The compiler should warn when a generic subprogram unit is withed but
not instantiated by the current main unit. This warning relies on flag
Is_Instantiated, which was wrongly set when the generic unit was also
withed and instantiated by some other unit.

This change merely reverts a fix done 20 years ago, which worked around
some spurious warning emitted with inlining enabled. Now the
Is_Instantiated flag is set similarly for generic subprograms and
generic packages.

gcc/ada/

	* atree.adb, atree.ads: Move WITH clause from spec to body; to
	prevent new warnings stopping the bootstrap.
	* fmap.adb, fname-sf.adb, libgnat/a-direct.adb,
	libgnat/s-bignum.adb: Remove unnecessary WITH clauses; to
	prevent new warnings stopping the bootstrap.
	* sem_ch12.adb (Analyze_Subprogram_Instantiation): Only set
	Is_Instantiated flag when the instance is in the extended main
	source unit.