[Ada] Internal error on iterator for limited private discriminated type

[Ada] Internal error on iterator for limited private discriminated type

This patch further extends the short-circuit, aka optimization, present
in the Check_Constrained_Object procedure used for renaming declarations
to all limited types, so as to prevent type mismatches downstream in
more cases.

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch8.adb (Check_Constrained_Object): Further extend the
	special optimization to all limited types.

gcc/testsuite/

	* gnat.dg/iter5.adb, gnat.dg/iter5_pkg.ads: New testcase.

From-SVN: r273677