[rustc_ty_utils] Add the LLVM noalias parameter attribute to drop_in_place in certain cases.

[pcwalton]

LLVM can make use of the noalias parameter attribute on the parameter to drop_in_place in areas like argument promotion. Because the Rust compiler fully controls the code for drop_in_place, it can soundly deduce parameter attributes on it. In the case of a value that has a programmer-defined Drop implementation, we know that the first thing drop_in_place will do is pass a pointer to the object to Drop::drop. Drop::drop takes &mut, so it must be guaranteed that there are no pointers to the object upon entering that function. Therefore, it should be safe to mark noalias there.

With this patch, we mark noalias only when the type is a value with a programmer-defined Drop implementation. This is possibly overly conservative, but I thought that proceeding cautiously was best in this instance.