[Ada] gprbuild fails to find ghost ALI files

[Ada] gprbuild fails to find ghost ALI files

This patch fixes a bug where if a ghost unit is compiled with
ignored-ghost mode in a library project, then gprbuild will fail to find
the ALI file, because the compiler generates an empty object file, but
no ALI file.

2019-07-01  Bob Duff  <duff@adacore.com>

gcc/ada/

	* gnat1drv.adb (gnat1drv): Call Write_ALI if the main unit is
	ignored-ghost.

From-SVN: r272867