[Ada] Expand Enum_Rep attribute reference in GNATprove mode

[Ada] Expand Enum_Rep attribute reference in GNATprove mode

In the special GNATprove mode for proof of programs, expand the Enum_Rep
attribute reference so that a suitable static integer is in the AST
where required by the rest of analysis.

There is no impact on compilation.

2019-07-09  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* exp_spark.adb (Expand_SPARK_N_Attribute_Reference): Expand
	attribute reference on Enum_Rep.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@273276 138bc75d-0d04-0410-961f-82ee72b054a4