- Add `LocalizationProducerState` to manage the states of LocalizationProducers.
- Add `initializeImpl` and `initializeIfNeeded` to manage lazily initialization of `LocalizationProducer`s.
- Move constructing a localization producer from DiagEngine to `LocalizationProrducer` itself.
Require producers to implement `getMessage(DiagID)` and
establish default implementation of `getMessageOr`, so
producers don't have to worry about handling fallback if
translation is unavailable.
Create a new diagnostics serialization library, splitting out of
swiftAST to reduce the overheads for building the tooling for
cross-compiling the toolchain. This should reduce the build time for
swift-serialize-diagnostics to enable cross-compilation.
