Ada: Fix bogus error on limited with clause and private parent package

Ada: Fix bogus error on limited with clause and private parent package

The implementation of the 10.1.2(8/2-11/2) subclauses that establish rules
for the legality of "with" clauses of private child units is done separately
for regular "with" clauses (in Check_Private_Child_Unit) and for limited
"with" clauses (in Check_Private_Limited_Withed_Unit).  The testcase, which
contains the regular and the "limited" version of the same pattern, exhibits
a disagreement between them; the former implementation is correct and the
latter is wrong in this case.

The patch fixes the problem and also cleans up the latter implementation by
aligning it with the former as much as possible.

gcc/ada/
	PR ada/34374
	* sem_ch10.adb (Check_Private_Limited_Withed_Unit): Use a separate
	variable for the private child unit, streamline the loop locating
	the nearest private ancestor, fix a too early termination of the
	loop traversing the ancestor of the current unit, and use the same
	privacy test as Check_Private_Child_Unit.

gcc/testsuite/
	* gnat.dg/specs/limited_with4.ads: Rename to...
	* gnat.dg/specs/limited_with1.ads: ...this.
	* gnat.dg/specs/limited_with4_pkg.ads: Rename to...
	* gnat.dg/specs/limited_with1_pkg.ads: ...this.
	* gnat.dg/specs/limited_with2-child1.ads: New test.
	* gnat.dg/specs/limited_with2-child2.ads: Likewise.
	* gnat.dg/specs/limited_with2.ads: New helper.