Previously we would only add @_disfavoredOverload if the only type
changed was the return type, because in any other case it is unambiguous
which overload to call. However it is still ambiguous when storing the
function as a value rather than calling the function, unless explicit
type annotations are used.
To avoid breaking any existing code, this patch adds
@_disfavoredOverload to every overload generated by @_SwiftifyImport.
rdar://151206394
(cherry picked from commit 0f312adb92)
f148b8d73b