Correctly print subrange types in generic_value_print

I noticed that generic_value_print assumes that a subrange type is
always a subrange of an integer type.  However, this isn't necessarily
the case.  In Ada, for example, one has subranges of character and
enumeration types.

This code isn't often exercised, I think, because languages with real
subrange types tend to implement their own printers.  However, it
still seemed worth fixing.