[Ada] Spurious tampering check failure

[Ada] Spurious tampering check failure

This patch modifies the transient scope mechanism to create a scope when the
condition of an iteration scheme returns a controlled result or involves the
secondary stack. As a result, a while loop which iterates over a container
properly manages the tampering bit at each iteration of the loop.

2018-05-31  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* exp_ch7.adb (Find_Transient_Context): An iteration scheme is a valid
	boudary for a transient scope.

gcc/testsuite/

	* gnat.dg/tampering_check1.adb, gnat.dg/tampering_check1_ivectors.ads,
	gnat.dg/tampering_check1_trim.adb, gnat.dg/tampering_check1_trim.ads:
	New testcase.

From-SVN: r261006