Tooling: Call back for both begin and end of sources

newFrontendActionFactory() took a pointer to a callback to call when a source
file was done being processed by an action. This revision updates the callback
to include an ante-processing callback as well.

Callback-providing class renamed and callback functions themselves renamed.
Functions are no longer pure-virtual so users aren't forced to implement both
callbacks if one isn't needed.

llvm-svn: 182864