Create a CaretDiagnostic class to hold the logic for emitting

Create a CaretDiagnostic class to hold the logic for emitting
(unsurprisingly) caret diagnostics. This is designed to bring some
organization to the monstrous EmitCaretDiagnostic function, and allow
factoring it more easily and with less mindless parameter passing.

Currently this just lifts the existing function into a method, and
splits off the obviously invariant arguments to be class members. No
functionality is changed, and there are still lots of warts to let
existing code continue functioning as-is. Definitely WIP, more cleanups
to follow.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138921 91177308-0d34-0410-b5e6-96231b3b80d8