This patch modifies the analysis (which is really expansion) of null
procedures to set the Ghost mode of the spec when the null procedure
acts as a completion.  This ensures that all nodes and entities
generated by the expansion are marked as Ghost, and provide a proper
context for references to Ghost entities.

2018-11-14  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_ch6.adb (Analyze_Null_Procedure): Capture Ghost and
	SPARK-related global state at the start of the routine. Set the
	Ghost mode of the completed spec if any.  Restore the saved
	Ghost and SPARK-related global state on exit from the routine.

gcc/testsuite/

	* gnat.dg/ghost1.adb, gnat.dg/ghost1.ads: New testcase.

From-SVN: r266132