This is the main PR of the Swift driver integration: synchronizes all swift-frontend invocations so emit-module (which is responsible to make the last check if the cached artifact can be used) is blocking swift-frontent -c (compilation) invocations.

Context

Because the design and implementation might not be obvious(relies on the swift driver integration in Xcode), they are documented in the markdown file (part of this PR), available to review here.

Previous PRs (#209 and #208)

Next steps

• Add swift-frontent executables to the released .zip package
• Add integration E2E tests
• Include support for integrations (both standalone and CocoaPods)

^ All these missing parts will be added in Part IV (Draft)