ada: Give error for reference to nonvisible library unit

ada: Give error for reference to nonvisible library unit

This patch fixes a bug where the compiler would allow
a name X to refer to a library unit that is not visible.
In particular, this happens when the name X occurs in the
private part of a library package, and the parent of that
package contains an instantiation of a generic package, and the
spec of that generic package has "private with X;",
but there is no "private with X;" or "with X;" that applies
to the place where the name X occurs.

Also misc cleanup.

gcc/ada/

	* sem_ch10.adb (Expand_With_Clause): Misc cleanup.
	(Install_Private_With_Clauses): Avoid installing a private
	with_clause that comes from an instantiated generic
	(it is marked as Implicit_With, but doesn't come from a parent
	with). Fix typo in comment, and other minor cleanups.