[Ada] gnatbind: ghost code with -gnatQ

[Ada] gnatbind: ghost code with -gnatQ

This patch fixes a bug where if a library unit is compiled with -gnatQ,
and that library unit is an ignored Ghost unit, then gnatbind silently
fails.

2018-12-11  Bob Duff  <duff@adacore.com>

gcc/ada/

	* gnat1drv.adb (gnat1drv): Pass the correct Object value when
	calling Write_ALI in the case of -gnatQ.
	* gnatbind.adb (Gnatbind): Avoid silent failure; give an error
	message.

From-SVN: r267004