The generality of the `AvailabilityContext` name made it seem like it
encapsulates more than it does. Really it just augments `VersionRange` with
additional set algebra operations that are useful for availability
computations. The `AvailabilityContext` name should be reserved for something
pulls together more than just a single version.
The goal is to have a lightweight way to pass an unapplied
diagnostic to general routines. Constructing a Diagnostic
is quite expensive as something we're potentially doing in
hot paths, as opposed to just when we're actually emitting
the diagnostic. This design allows the expense to be delayed
until we need it.
I've also optimized the Diagnostic constructor to avoid
copying arguments unnecessarily; this is a relatively small
expense, since arguments are POD, but there's really no good
reason not to do it.
