[Ada] Adapt GNATprove expansion for slices with access prefix

[Ada] Adapt GNATprove expansion for slices with access prefix

The special expansion done in GNATprove mode should be adapted to slices
where the prefix has access type, like indexed expressions.

There is no impact on compilation.

2019-08-20  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* exp_spark.adb (Expand_SPARK_N_Slice_Or_Indexed_Component):
	Renaming of function to apply to slices as well.
	(Expand_SPARK): Expand prefix of slices of access type.

From-SVN: r274740