Adds a dataflow analysis that detects unsafe accesses to values of type
`std::optional`, `absl::optional`, or `base::Optional`.
Reviewed-by: ymandel, xazax.hun
Differential Revision: https://reviews.llvm.org/D121197
Users outside of the clang repo may use different googletest versions. So, it's
better not to depend on llvm's googletest. This patch removes the dependency by
having `checkDataflow` return an `llvm::Error` instead of calling googletest's
`FAIL` or `ASSERT...` macros.
Differential Revision: https://reviews.llvm.org/D117304
Currently, the transfer function returns a new lattice element, which forces an
unnecessary copy on processing each CFG statement.
Differential Revision: https://reviews.llvm.org/D116834
