Cast_ptr_alignment false negative in trait method default impl

[dtolnay]

I believe Clippy should catch both of the following casts but it currently only flags the first one.

unsafe fn frob(bytes: *const u8) -> *const String {
    // Correctly caught by cast_ptr_alignment.
    bytes as *const String
}

trait Trait {
    unsafe fn frob(bytes: *const u8) -> *const Self where Self: Sized {
        // Should be caught but is not.
        bytes as *const Self
    }
}

Would have prevented this bug in Chrome OS: https://bugs.chromium.org/p/chromium/issues/detail?id=900962.

clippy 0.0.212 (f5d868c 2018-11-15)