[Ada] Improve warnings about infinite loops

[Ada] Improve warnings about infinite loops

The compiler now has fewer false alarms when warning about infinite
loops. For example, a loop of the form "for X of A ...", where A is an
array, cannot be infinite.  The compiler no longer warns in this case.

2019-07-03  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_warn.adb (Check_Infinite_Loop_Warning): Avoid the warning
	if an Iterator_Specification is present.

gcc/testsuite/

	* gnat.dg/warn20.adb, gnat.dg/warn20_pkg.adb,
	gnat.dg/warn20_pkg.ads: New testcase.

From-SVN: r272978