ada: Missing support for consistent assertion policy

Add missing support for RM 10.2/5: the region for a pragma
Assertion_Policy given as a configuration pragma is the
declarative region for the entire compilation unit (or units)
to which it applies.

gcc/ada/

	* sem_ch10.adb (Install_Inherited_Policy_Pragmas): New subprogram.
	(Remove_Inherited_Policy_Pragmas): New subprogram.
	(Analyze_Compilation_Unit): Call the new subprograms to
	install and remove inherited assertion policy pragmas.