Frontend: Factor out header include dumping (-H) into its own preprocessor

Frontend: Factor out header include dumping (-H) into its own preprocessor
callbacks class.
 - Aside from being generally cleaner, this also allows -H to work correctly in
   modes other than standard preprocessing (e.g., -c, -MM, etc.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124723 91177308-0d34-0410-b5e6-96231b3b80d8