Rename isInCurrentDSO -> isInCurrentOutput.

DSO is short for dynamic shared object, so the function name was a
little confusing because it sounded like it didn't work when we were
a creating statically-linked executable or something.

What we mean by "DSO" here is the current output file that we are
creating. Thus the new name. Alternatively, we could call it the current
ELF module, but "module" is a overloaded word, so I avoided that.

llvm-svn: 316809