[Sema] Allow implicit conversions of &overloaded_fn in C.

Also includes a minor ``enable_if`` docs update.

Currently, our address-of overload machinery will only allow implicit
conversions of overloaded functions to void* in C. For example:

```
void f(int) __attribute__((overloadable));
void f(double) __attribute__((overloadable, enable_if(0, "")));

void *fp = f; // OK. This is C and the target is void*.
void (*fp2)(void) = f; // Error. This is C, but the target isn't void*.
```

This patch makes the assignment of `fp2` select the `f(int)` overload,
rather than emitting an error (N.B. you'll still get a warning about the
`fp2` assignment if you use -Wincompatible-pointer-types).

Differential Revision: http://reviews.llvm.org/D13704

llvm-svn: 264132