[analyzer] Introduce a new callback for checkers, printState, to be used for...

[analyzer] Introduce a new callback for checkers, printState, to be used for debug-printing the contents of a ProgramState.

Unlike the other callbacks, this one is a simple virtual method, since it is only to be used for debugging.

This new callback replaces the old ProgramState::Printer interface, and allows us to move the printing of refcount bindings from CFRefCount to RetainReleaseChecker.

llvm-svn: 138728