Sink the non-caret diagnostic logic down and out of the diagnostic

consumer. The TextDiagnostic interface now has a generic entry point for
emitting a diagnostic which uses a minimal interface that should be
compatible with StoredDiagnostics such as are available in libclang etc.

Some unfortunate shuffling of static functions as things get relocated.
Also some unfortunate public interface points added to
TextDiagnosticPrinter, but those are the next bits to get moved so they
won't last long.

llvm-svn: 142064