[DebugInfo] Turn DWARFContext::Section into DWARFSection (NFC).

It would be more convenient to pass DWARFSection into DWARFUnitSection
constructor, instead of passing its components (Data and RelocAddrMap)
as a separate arguments.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219252 91177308-0d34-0410-b5e6-96231b3b80d8