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.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182864 91177308-0d34-0410-b5e6-96231b3b80d8