ada: Implement use implies with experimental extension

ada: Implement use implies with experimental extension

The patch implements the experimental feature to allow use package
clauses within the context area to imply with.

gcc/ada/ChangeLog:

	* sem_ch8.adb (Analyze_Package_Name): Add code to expand use
	clauses such that they have an implicit with associated with them
	when extensions are enabled.
	* sem_ch10.ads (Analyze_With_Clause): New.
	* sem_ch10.adb (Analyze_With_Clause): Add comes from source check
	for warning.
	(Expand_With_Clause): Moved to the spec.
	* sem_util.adb, sem_util.ads
	(Is_In_Context_Clause): Moved from sem_prag.
	* sem_prag.adb (Analyze_Pragma): Update calls to
	Is_In_Context_Clause.
	(Is_In_Context_Clause): Moved to sem_util.