The frontend should not know or care which array ops are implemented in druntime

@ibuclaw @klickverbot @redstar

Is this going to cause problems for the other backends?

Essentially it just skips emitting the function if it knows it's already defined in the backend.  The calling code has a reference to the `FuncDeclaration` that was never emitted, where before it referenced a body-less `FuncDeclaration` created by `getCfunc`.

The Digital Mars backends doesn't care.