Any constraints which would previously cause binding inference to
fail should instead delay associated type variable and preserve
all of the collected information.
This is vital for incremental binding computation that cannot
re-introduce constraints after "failure" because it's too expensive.
Create a new namespace - `swift::constraints::inference` and associate
`PotentialBinding` with it. This way it would be possible for constraint
graph to operate on `PotentialBinding(s)` in the future.
