This is the first patch in a series of patches for the IR change
tracking component of SandboxIR.
The tracker collects changes in a vector of `IRChangeBase` objects and
provides a `save()`/`accept()`/`revert()` API.
Each type of IR changing event is captured by a dedicated subclass of
`IRChangeBase`. This patch implements only one of them, that for
updating a `sandboxir::Use` source value, named `UseSet`.
