Rationalize the last bit of "arbitrary" state that is carried between

diagnostics to control suppression of redundant information. It now
follows the same model as all the other state, and has a bit more clear
semantics.

This is making the duality of the state a bit annoying, and I've added
a FIXME to resolve it. The problem is that I need to lift the
TextDiagnostic up into an externally visible layer before that can
happen.

llvm-svn: 142083