ada: Missing runtime check in interpolated string

ada: Missing runtime check in interpolated string

When the type imposed by the context for an interpolated string is
constrained, the compiler silently omits adding a runtime check.

gcc/ada/ChangeLog:

	* exp_ch2.adb (Expand_N_Interpolated_String_Literal): Use the
	base type of the type imposed by the context for building the
	interpolated string image; required to allow the expander adding
	the missing runtime check when the target type is constrained.
	(Apply_Static_Length_Check): New subprogram.