In CodePeer mode attribute Image is not expanded and has its Etype set
as the unconstrained String type. When this attribute appears as a
prefix of an indexed component, we get a check; when it appears as a
prefix of a slice, we don't get a check.

For indexed components, the check effectively comes from
Apply_Scalar_Range_Check, where prefixes of unconstrained array types
are handled specifically. For slices, the seemingly similar routine
Apply_Range_Check doesn't specifically handle such prefixes. Instead, we
need to give this routine a constrained subtype, just like we do when
slice is prefixed with a call to a function that returns an
unconstrained array.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_res.adb (Resolve_Slice): Add custom handling of attribute
	Image and similar in CodePeer mode. This complements the
	existing custom handling of these attributes in
	Expand_N_Attribute_Reference.