[clang-format] Fix support for ObjC blocks with pointer return types

The ObjC-block detection code only supports a single token as the return type. Add support to detect pointers, too (ObjC has lots of object-pointers).

For example, using `BasedOnStyle: WebKit`, the following is stable output:

```
int* p = ^int*(void)
{ //
    return nullptr;
}
();
```

After the patch, this is stable:

```
int* p = ^int*(void) { //
    return nullptr;
}();
```

Differential Review: https://reviews.llvm.org/D146434