[Ada] Ignore missing ALI files in GNATprove mode

[Ada] Ignore missing ALI files in GNATprove mode

This change only affects GNATprove backend, where it avoids spurious
errors on missing ALI files for units indirectly withed from the current
unit and processed as part of a different .gpr project.

No test provided, because only GNATprove is affected.

2019-09-17  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* ali.ads: Fix casing in comment.
	* ali-util.ads, ali-util.adb (Read_Withed_ALIs): Remove
	Ignore_Errors parameter; it was only set to non-default value of
	True when running in GNATprove_Mode and wrongly reset to False
	when calling this routine recursively. Now in GNATprove mode we
	want it to be always True, so in fact it is equivalent to
	GNATProve_Mode flag itself (which was already used in this
	routine).

From-SVN: r275775