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

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not-yet-built or out-of-date module.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>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169057 91177308-0d34-0410-b5e6-96231b3b80d8