[Basic] Change DiagnosticBuilder to use a separate status variable to track...

[Basic] Change DiagnosticBuilder to use a separate status variable to track whether the builder is active.
 - This may seem superflous, but actually this allows the optimizer to more
   easily eliminate the isActive() checks needed by the SemaDiagnosticBuilder
   and DiagnosticBuilder dtors. And by more easily, I mean the current LLVM is
   actually able to do one and not the other. :)

This is good for another 20k code size reduction.

llvm-svn: 152709