[Ada] Improve warnings about infinite loops

[Pierre-Marie de Rodat]

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.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.diff
Type: text/x-diff
Size: 1778 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20190703/a4f3cb80/attachment.bin>