[Ada] Update FE check following change in SPARK RM 7.1.3(12)

[Ada] Update FE check following change in SPARK RM 7.1.3(12)

SPARK Reference Manual changed to accept attributes First, Last and Length
as not leading to an evaluation of a part of the prefix object. This is
reflected here in the checking code for that rule.

2018-05-28  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_util.adb (Is_OK_Volatile_Context): Add attributes First, Last and
	Length as valid non-interfering contexts for SPARK.

From-SVN: r260836