[openacc] Rename generated file from ACC.cpp.inc to ACC.inc to match D92955

This patch rename the tablegen generated file ACC.cpp.inc to ACC.inc in order
to match what was done in D92955. This file is included in header file as well as .cpp
file so it make more sense.

Reviewed By: sameeranjoshi

Differential Revision: https://reviews.llvm.org/D93485