Push the CURangeList down into the skeleton CU (where available) rather than the full CU

So that it may be shared between skeleton/full compile unit, for CU
ranges and other ranges to be added for fission+gmlt.

(at some point we might want some kind of object shared between the
skeleton and full compile units for all those things we only want one of
in that scope, rather than having the full unit always look through to
the skeleton... - alternatively, we might be able to have the skeleton
pointer (or another, separate pointer) point to the skeleton or to the
unit itself in non-fission, so we don't have to special case its
absence)

llvm-svn: 221186