Start work on SerializedDiagnosticPrinter, a new DiagnosticConsumer that...

Start work on SerializedDiagnosticPrinter, a new DiagnosticConsumer that serializes out the diagnostics for a given translation unit to a bit code file.  This is a WIP.

The motivation for this new DiagnosticConsumer is to provide a way for tools invoking the compiler
to get its diagnostics via a libclang interface, rather than textually parsing the compiler output.
This gives us flexibility to change the compiler's textual output, but have a structured data format
for clients to use to get the diagnostics via a stable API.

I have no tests for this, but llvm-bcanalyzer so far shows that the emitted file is well-formed.

More work to follow.

llvm-svn: 143259