Extract the emission of the diagnostic's location into a separate

Extract the emission of the diagnostic's location into a separate
function. This is really the beginning of the second phase of
refactorings here. The end goal is to have (roughly) three interfaces:

1) Base class to format a single diagnostic suitable for display on the
   console.
2) Extension of the base class which also displays a caret diagnostic
   suitable for display on the console.
3) An adaptor that implements the DiagnosticClient by delegating to #1
   and/or #2 as appropriate.

Once we have these, things like libclang's formatDiagnostic can use #1
and #2 to provide really well formatted (and consistently formatted!)
textual formatting of diagnostics.

Getting there is going to be quite a bit of shuffling. I'm basically
sketching out where the interface boundaries can be drawn for #1 and #2
within the existing classes. That lets me shuffle with a minimum of fuss
and delta. Once that's done, and any of the related interfaces that need
to change are updated, I'll hoist these into separate headers and
re-implement libclang in terms of their interfaces. Long WIP, but
comments at each step welcome. =D

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139228 91177308-0d34-0410-b5e6-96231b3b80d8