sem_ch4.adb (Operator_Check): improve error message when both a with_clause and a...

sem_ch4.adb (Operator_Check): improve error message when both a with_clause and a use_clause are needed to...

2011-08-01  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch4.adb (Operator_Check): improve error message when both a
	with_clause and a use_clause are needed to make operator usage legal.
	* sem_util.ads, sem_util.adb (Unit_Is_Visible): new predicate to
	determine whether a compilation unit is visible within an other,
	either through a with_clause in the current unit, or a with_clause in
	its library unit or one one of its parents.

From-SVN: r177033