[Attributor] Add support for compound assignment for ChangeStatus

A common use of `ChangeStatus` is as follows:
```
ChangeStatus Changed = ChangeStatus::UNCHANGED;
Changed |= foo();
```
where `foo` returns `ChangeStatus` as well. Currently `ChangeStatus` doesn't
support compound assignment, we have to write as
```
Changed = Changed | foo();
```
which is not that convenient.

This patch add the support for compound assignment for `ChangeStatus`. Compound
assignment is usually implemented as a member function, and binary arithmetic
operator is therefore implemented using compound assignment. However, unlike
regular C++ class, enum class doesn't support member functions. As a result, they
can only be implemented in the way shown in the patch.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106109