Introduce a constructor that takes an `llvm::VersionTuple` directly, instead of
needing to spell out `VersionRange::allGTE(<tuple>)` which is unnecessarily
verbose.
One to get the active domain for the compilation target and another to get the
ABI compatibility domain for a given domain. The ABI compatibility domain will
be needed for queries that compute whether an unavailable declaration is still
reachable at runtime.
NFC.
