Use correct sign extension for enumeration types

This changes update_enumeration_type_from_children to use the correct
sign-extension method on the attribute.  The logic here is a bit
complicated: if the enum has an underlying type, then we use that
type's signed-ness to interpret attributes; otherwise we must assume
attributes are encoded as signed values.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680