Split DWARF: Allow -gmlt/-gsplit-dwarf to override rather than complement each other

It doesn't make any sense to enable -gmlt with -gsplit-dwarf, since
-gmlt is designed for on-line symbolication (and -gsplit-dwarf normally
emits all the -gmlt data into the .o anyway - so there's nothing to
split out except redundant/duplicate info).

With this change they override each other, -gmlt -gsplit-dwarf is the
same as -gsplit-dwarf and -gsplit-dwarf -gmlt is the same as -gmlt.

llvm-svn: 243694