DebugInfo: Fix -gsplit-dwarf + -fno-split-dwarf-inlining

I tested the cases involving split-dwarf + gmlt +
no-split-dwarf-inlining, but didn't verify the simpler case without
gmlt.

The logic is, admittedly, a little hairy, but seems about as simple as I
could wrangle it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280290 91177308-0d34-0410-b5e6-96231b3b80d8