Make changes to SDiagsWriter to make it work in combination with the ARC migrator:

-Allow it to be used with multiple BeginSourceFile/EndSourceFile calls; for this introduce
 a "finish" callback method in the DiagnosticConsumer. SDiagsWriter finishes up the serialization
 file inside this method.
-Make it independent of any particular DiagnosticsEngine; make it use the SourceManager of the
 Diagnostic object.
-Ignore null source ranges.

llvm-svn: 146020