ada: Fix SPARK context not restored when Load_Unit is failing

ada: Fix SPARK context not restored when Load_Unit is failing

When Load_Unit fails to find the unit or encounters an error, the
Load_Fail procedure is called and an exception is raised, skipping the
restoration of the SPARK/Ghost context stored on procedure entry.

gcc/ada/

	* rtsfind.adb (Load_RTU.Restore_SPARK_Context): New.
	(Load_RTU): Use Restore_SPARK_Context on all exit paths.
	* sem_ch6.adb (Analyze_Subprogram_Body_Helper): Initialize local
	variable to Empty.