ada: Correction to disable self-referential with_clauses

ada: Correction to disable self-referential with_clauses

Follow-on to previous change "Disable self-referential with_clauses",
which caused some regressions. Remove useless use clauses referring
to useless self-referential with'ed packages. This is necessary
because in some cases, such use clauses cause the compiler to
crash or give spurious errors.

In addition, enable the warning on self-referential with_clauses.

gcc/ada/ChangeLog:

	* sem_ch10.adb (Analyze_With_Clause): In the case of a
	self-referential with clause, if there is a subsequent use clause
	for the same package (which is necessarily useless), remove it from
	the context clause. Reenable the warning.