Support index-while-building when building with Bazel

[brentleyjones]

When using the swift.index_while_building feature, Bazel will produce index stores while building. We should enable that feature by default in _xcodeproj_transition (though once we have multiple configs, we should only enable it for build, not index) and:

• Add the outputs to the build output group
• Use index-import to copy the index files into Xcode's index store
  • We will want to use -import-output-file to only copy over what's needed for each library
  • Though if using swift.use_global_index_store, the per-library index store should already be only what's needed? If so, we could pass that feature as well/instead
  • If we do this based on the need to run stubs/swiftc.py, then we can reduce the amount of copying further, as stubs/swiftc.py will only run when something has changed

[brentleyjones]

With Hybrid BwX, and eventually Focused Projects, the need for this is going up.

[brentleyjones]

Non-swift targets will probably need https://github.com/bazelbuild/bazel/issues/15983.

[brentleyjones]

Without this there is no indexing data for unfocused targets. Even with that, I'm removing it as a blocker for 0.7.0, as there are performance considerations I want to deeply investigate, including being able to remove index-while-building from the Index Build and SwiftUI Previews builds.

[brentleyjones]

https://github.com/buildbuddy-io/rules_xcodeproj/issues/1060

[brentleyjones]

#1040 resolved this.