swift234

[ClangImporter] Import C function types as CFunctionPointer<T>.

...where T is the equivalent Swift function type. This gives us proper type
safety (ish) for C function pointers while still not treating them the same
as Swift closures.

If the function type is not one we can represent in Swift, we fall back to
COpaquePointer. CFunctionPointer<T> and COpaquePointer can be explicitly
constructed from one another, but do not freely convert.

<rdar://problem/17215978>

Swift SVN r19154