Kick JSON output for XML output.

Apparently the owners of the tools we want to integrate with (eclipse in
this case) don't have JSON parsers.

The output now is:
<replacements>
<replacement offset='2' length='3'>  </replacement>
...
</replacements>

Kicking JSON for now - it's easy enough to get back in when we need it.

FIXME: once we find this useful enough, we might want to add it as
free-standing functions to tooling.

llvm-svn: 174497