swift234

[build-script] Switch over to computing targets in `build-script`.

 - Introduces a new `HostSpecificConfiguration` which holds the computed
   information for each host we need to use.

 - This relies on the functionality to pass host-specific variables to the
   script via environment variables.

 - The computation itself has been cleaned up a tiny bit (mostly via using
   the factored out platform/target properties), but is otherwise a straight
   port of the logic from `build-script-impl`.

 - This commit does not yet remove that code from `build-script-impl`; instead
   it validates that the two implementations are computing the same result.
   This is useful right now for testing the validity of the port.