[dsymutil] Add support for linking line tables.

This code comes with a lot of cruft that is meant to mimic darwin's
dsymutil behavior. A much simpler approach (described in the numerous
FIXMEs that I put in there) gives the right output for the vast
majority of cases. The extra corner cases that are handled differently
need to be investigated: they seem to correctly handle debug info that
is in the input, but that info looks suspicious in the first place.

Anyway, the current code needs to handle this, but I plan to revisit it
as soon as the big round of validation against the classic dsymutil is
over.

llvm-svn: 232333