Rename some classes in the AST package from A* to Abstract*
These are the classes that should never be referenced except by their
subclasses.
This allows to use consistent A* names for all actually-relevant types.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14491 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c