ada: Introduce Cannot_Be_Superflat flag on N_Range nodes

ada: Introduce Cannot_Be_Superflat flag on N_Range nodes

The support of superflat arrays in the language generates an overhead that
the code generator attempts to minimize, but it cannot handle too complex
cases and it would be helpful if the front-end could lend a hand.

This change introduces the Cannot_Be_Superflat flag on N_Range nodes for
this purpose, and sets it on the result of string concatenations when it
is guaranteed to be nonnull.

gcc/ada/

	* gen_il-fields.ads (Opt_Field_Enum): Add Cannot_Be_Superflat.
	* gen_il-gen-gen_nodes.adb (N_Range): Add Cannot_Be_Superflat as
	semantical flag and change Includes_Infinities to semantical.
	* sinfo.ads (Cannot_Be_Superflat): Document it for N_Range.
	* exp_ch4.adb (Expand_Concatenate): Set Cannot_Be_Superflat on the
	range of the result if the result cannot be null.