[Ada] Retention of with clauses for ignored Ghost units

[Ada] Retention of with clauses for ignored Ghost units

This patch ensures that with clauses that mention ignored Ghost units are
retained in the tree. The retention is necessary for several reasons:

   * The with clauses allow the new elaboration order mechanism to
     produce the same library edges regardless of whether the Ghost unit
     is checked or ignored. This ensures that the elaboration order
     remains consistent.

   * The with clauses allow the unnesting mechanism to properly
     recognize that all units have been examined for unnesing purposes.

No observable impact, no test needed.

2018-08-21  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_ch10.adb: Remove the with and use clause for unit Ghost.
	(Analyze_With_Clause): Do not mark a with clause which mentions
	an ignored Ghost code for elimination.

From-SVN: r263733