Typeck: Disallow scalar casts to bare_fn.

Bare functions are another example of a scalar but non-numeric
type (like char) that should be handled separately in casts.
This disallows expressions like `0 as extern "Rust" fn() -> int;`.

It might be advantageous to allow casts between bare functions
and raw pointers in unsafe code in the future, to pass function
pointers between Rust and C.

Closes #8728