[Ada] Expansion of discrete choices

[Ada] Expansion of discrete choices

This patch does some minor bookkeeping to avoid a potential double expansion
of discrete choices where at least one of them is a subtype with predicates.
No change in behavior, no need for a test.

2018-05-24  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* exp_util.adb (Expand_Static_Predicates_In_Choices): Indicate that the
	construct with discrete choices no longer contains a subtype with
	predicates since the expansion already handled this case.

From-SVN: r260646