-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