When defining a C-compatible function with `@c`, we were emitting a
Swift function along with a C-compatible thunk. Stop doing that, and
instead only produce the C-compatible function. All uses of the
function will go through that C interface, just like if the function
were declared in C.
This also applies to `@c @implementation` functions, which are
declared in C but implemented in Swift. It does *not* apply to
`@_cdecl`, which will continue to produce both the Swift function and
C thunk to prevent an ABI break.
Fixes rdar://158888024.