[clang][dataflow] Add base types for building dataflow analyses This is part of the implementation of the dataflow analysis framework. See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev. Reviewed By: ymandel, xazax.hun, gribozavr2 Differential Revision: https://reviews.llvm.org/D114234 GitOrigin-RevId: 6e2aecd759dc3f8bf664323388365ac234271e9b