[clang][dataflow] Remove TestingSupport's dependency on gtest

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