[Ada] Faulty ignored Ghost code removal

[Ada] Faulty ignored Ghost code removal

This patch ensures that removal of ignored Ghost code is the absolute last
operation performed on the tree. Previously the removal was performed prior to
issuing delayed warnings, however the warning mechanism may see a heavily
modified tree and fail.

No small reproducer available.

2018-07-17  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* frontend.adb (Frontend): The removal of ignored Ghost code must be
	the last semantic operation performed on the tree.

From-SVN: r262795