[Ada] Crash on incorrect frontend inlining in GNATprove mode

[Ada] Crash on incorrect frontend inlining in GNATprove mode

In some cases, the inlining performed in GNATprove mode leads to a crash,
when inlining a call where a return statement of the inlined function
returns a string literal. Now fixed.

2018-05-21  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_eval.adb (Static_Length): Take into account case of variable of
	subtype string literal.

From-SVN: r260452