[Ada] Avoid namespace pollution for Next and Previous

[Ada] Avoid namespace pollution for Next and Previous

This patch renames Next and Previous in a-convec.ads and other
containers to be _Next and _Previous, to avoid namespace pollution.  The
compiler now uses the leading-underscore names to look them up.

The scanner is changed to allow this.

gcc/ada/

	* exp_ch5.adb (Expand_Iterator_Loop_Over_Array): Use _Next and
	_Previous in the optimized expansion of "for ... of".  No longer
	need to check parameter profiles for these, because the
	leading-underscore names are unique.
	* libgnat/a-convec.ads (_Next, _Previous): Renamings of Next and
	Previous, to avoid namespace pollution.
	* libgnat/a-cbdlli.ads, libgnat/a-cbhama.ads,
	libgnat/a-cbhase.ads, libgnat/a-cbmutr.ads,
	libgnat/a-cborma.ads, libgnat/a-cborse.ads,
	libgnat/a-cdlili.ads, libgnat/a-cidlli.ads,
	libgnat/a-cihama.ads, libgnat/a-cihase.ads,
	libgnat/a-cimutr.ads, libgnat/a-ciorma.ads,
	libgnat/a-ciorse.ads, libgnat/a-cobove.ads,
	libgnat/a-cohama.ads, libgnat/a-cohase.ads,
	libgnat/a-coinve.ads, libgnat/a-comutr.ads,
	libgnat/a-coorma.ads, libgnat/a-coorse.ads: Likewise.  Also,
	remove duplicated comments -- refer to one comment about _Next,
	_Previous, Pseudo_Reference in libgnat/a-convec.ads. DRY.
	* scng.adb (Scan): Allow leading underscores in identifiers in
	the run-time library.
	* snames.ads-tmpl (Name_uNext, Name_uPrevious): New names with
	leading underscores.