[clang] [clang][dataflow] Emit an error if source code is not compiled as C++. (PR #65301)

[Cody Tapscott via cfe-commits]

topolarity wrote:

Is this the right place to add this check? It doesn't seem like there's any C++-specific logic here.

Presumably the real incompatibility is `clang::CFG` or one of the consumers of `dataflow::ControlFlowContext`?

https://github.com/llvm/llvm-project/pull/65301