Run C++ control flow validation on FunctionDefs before running.

Run C++ control flow validation on FunctionDefs before running.

Clients should ideally prevent such functions from being created in
the first place, but we still want the runtime to be robust to
malformed functions. Trying to run functions with invalid control flow
constructs can result in crashes or hangs, so we want to catch it
before running.

PiperOrigin-RevId: 180727589