Teach the serialized diagnostic writer to clone() itself, sharing

state so that all of the various clones end up rendering their
diagnostics into the same serialized-diagnostics file. This is
important when we actually want failures during module build to be
reported back to the translation unit that tried to import the
not-yet-built or out-of-date module. <rdar://problem/12565727>

llvm-svn: 169057