Replacing Hello World example Plugin with one that counts and prints the names of
functions and subroutines.
This involves changing the `PluginParseTreeAction` Plugin base class to
inherit from `PrescanAndSemaAction` class to get access to the Parse Tree
so that the Plugin can walk it.
Additionally, there are tests of this new Plugin to check it prints the correct
things in different circumstances.
Depends on: D106137
Reviewed By: awarzynski
Differential Revision: https://reviews.llvm.org/D107089
Stuart Ellis