cannot use an `as` expression to coerce, so I used a one-off function
instead (this is a no-op in stage0, but in stage1+ it triggers
coercion from the fn pointer to the fn item type).
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
