Sink the logic for suppressing some macro expansion notes from the

TextDiagnosticPrinter into the CaretDiagnostic class. Several
interesting results from this:

- This removes a significant per-diagnostic bit of state from the
  CaretDiagnostic class, which should eventually allow us to re-use the
  object.
- It removes a redundant recursive walk of the macro expansion stack
  just to compute the depth. We don't need the depth until we're
  unwinding anyways, so we can just mark when we reach it.
- It also paves the way for several simplifications we can do to how we
  implement the suppression.

llvm-svn: 140474