DebugInfo: Test & handle (differently) non-zero DW_AT_ranges_base

Followup to r309570, fixing it slightly differently (ranges_base and
addr_base should never be read from a DWO file - so there shouldn't be
any issue with 'overriding' the values - conditionalize the code and
assert that the values aren't being overriden).

llvm-svn: 309879