ada: Incorrect accessibilty level for library level subprograms

The patch fixes an issue in the compiler whereby accessibility level
calculations for objects declared witihin library-level subprograms
were done incorrectly - potentially allowing runtime accessibility
checks to spuriously pass.

gcc/ada/ChangeLog:

	* accessibility.adb:
	(Innermost_master_Scope_Depth): Add special case for expressions
	within library level subprograms.