[SemaObjC] Forbid storing an unboxed integer literal in an NSNumber

This fixes a common mistake (the 3 should be @3): NSNumber *n = 3. This extends
an existing check for NSString. Also, this only errs if the initializer isn't a
null pointer constant, so NSNumber *n = 0; continues to work. rdar://47029572

Differential revision: https://reviews.llvm.org/D78066