[analyzer] Nullability: Suppress diagnostic on bind with cast.

Update the nullability checker to allow an explicit cast to nonnull to
suppress a warning on an assignment of nil to a nonnull:

id _Nonnull x = (id _Nonnull)nil; // no-warning

This suppression as already possible for diagnostics on returns and
function/method arguments.

rdar://problem/25381178

llvm-svn: 266219