DebugInfo: Let -gsplit-dwarf and -gmlt compose if -fno-split-dwarf-inlining is used

If the inline info is not duplicated into the skeleton CU, then there's
value in using -gsplit-dwarf and -gmlt together (to keep all those extra
subprograms out of the skeleton CU, while also producing smaller .dwo
files)

llvm-svn: 279687