[CodeGen/Dwarf] Make debug_names compatible with split-dwarf

Summary:
Previously we crashed for the combination of the two features because we
tried to reference the dwo CU from the main object file. The fix
consists of two items:
- reference the skeleton CU from the name index (the consumer is
  expected to use the skeleton CU to find the real data).
- use the main object file string pool for the strings in the index

Reviewers: JDevlieghere, aprantl, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45566

llvm-svn: 330249