GPGPU: Add dummy implementation for ast expression construction

Instead of calling to a pet function that does not return anything, we pass
our own dummy implementation to ppcg that always returns a nullptr. This
ensures that the list of ast expressions always contains a nullptr and we do
not accidentally free a random (uninitalized) pointer. This resolves the
last valgrind warning we see.

We provide an implementation for this function, when the generated AST
expressions can be used and consequently can be tested.

llvm-svn: 275435