[Ada] Expand type of static expressions in GNATprove mode

[Ada] Expand type of static expressions in GNATprove mode

In the special mode for GNATprove, expand the type of static expressions
like done during compilation, to both get suitable legality checks and
increase the precision of the formal analysis.

There is no impact on compilation.

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

gcc/ada/

	* exp_util.adb (Expand_Subtype_From_Expr): Still expand the type
	of static expressions in GNATprove_Mode.
	* sem_ch3.adb (Analyze_Object_Declaration): Remove obsolete
	special case for GNATprove_Mode.

From-SVN: r273273