[Ada] Fix internal error on array slice in loop and Loop_Invariant

[Ada] Fix internal error on array slice in loop and Loop_Invariant

This fixes an internal error caused by the presence of an Itype in a
wrong scope.  This Itype is created for an array slice present in the
condition of a while loop whose body also contains a pragma
Loop_Invariant, initially in the correct scope but then relocated into a
function created for the pragma.

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

gcc/ada/

	* exp_attr.adb (Expand_Loop_Entry_Attribute): Copy the condition
	of a while loop instead of simply relocating it.

gcc/testsuite/

	* gnat.dg/loop_invariant1.adb, gnat.dg/loop_invariant1.ads: New
	testcase.

From-SVN: r273668