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.

llvm-svn: 280290