MONGO_COMPILER_UNREACHABLE should terminate if violated

The current MONGO_COMPILER_UNREACHABLE simply uses __builtin_unreachable, however if control does flow past it then the behavior is undefined.

We would prefer to terminate. We should write a new macro MONGO_UNREACHABLE, that invokes invariantFailed or similar. Optionally, the macro could follow that call to invariantFailed with __builtin_unreachable, however this should probably be unnecessary, as whatever function is called by the macro should be tagged noreturn.