[Ada] Unnesting: fix for constrained arrays and improve static constants

[Ada] Unnesting: fix for constrained arrays and improve static constants

2018-12-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (Needs_Fat_Pointer): A fat pointer is required if
	the entity has a private type whose full view is an
	unconstrained array type.
	(Rewrite_One_Ref): If the reference is to a static constant, use
	its value rather than create a reference through the activation
	record. This is more efficient, and furthermore indispensable if
	the context requires a static constant, such as in a branch of a
	case statement.

From-SVN: r267003