[Ada] Exit statement in loops over iterable objects

[Ada] Exit statement in loops over iterable objects

This patch fixes an omission in the expansion of loops over GNAT-specific
iterable objects. If the source includes an explicit name for the loop,
that name has to be preserved in the expanded code to allow exit statements
to mention it.

2018-05-21  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch5.adb (Build_Formal_Container_Iteration): If source has
	explicit name for iterator loop, preserve that name in expanded
	construct, for possible use in exit statements.

gcc/testsuite/

	* gnat.dg/exit1.adb: New testcase.

From-SVN: r260465