Revert r198199 (and r198202). It broke 3 DebugInfo tests for targeting i686-cygming.

  r198196: Use a pointer to keep track of the skeleton unit for each normal unit and construct it up front.
  r198199: Reapply r198196 with a fix to zero initialize the skeleton pointer.
  r198202: Fix aranges and split dwarf by ensuring that the symbol and relocation back to the compile unit from the aranges section is to the skeleton unit and not the one in the dwo.

They could be reproducible with explicit target.

  llvm/lib/MC/WinCOFFObjectWriter.cpp:224: bool {anonymous}::COFFSymbol::should_keep() const: Assertion `Section->Number != -1 && "Sections with relocations must be real!"' failed.