As it is highly possible that Xcode 15 beta (expected on in a week, on Jun 5th) will no longer support SWIFT_USE_INTEGRATED_DRIVER: NO, we should be ready to add support for the integrated driver.

This is a first PR that refactors the existing implementation so it the swift-driver PR will be easier to reason about.

This PR doesn't change anything in the business logic.

Includes:

• Adds a mode in the BuildSettingsIntegrateAppender to specify if the SWIFT_USE_INTEGRATED_DRIVER should be added or not (for now, always add)
• Splits Swiftc into 2 phases: emit module and compilation. With swiftc, this is always done in a single process (at least from XCRemoteCache's perspective), in the driver world, that will be 2 independent processes
• Adds extra options for the input (*.swift) and output (*.d, *.o etc) files: either via a file (used currently) or explicitly in the cmd argument list
• XCSwiftc's logic is broken down to the XCSwiftAbstract, which will be shared with the XCSwiftFrontend
• Making some params optional, which are nil in the swift-frontend

Next steps

Add swift-frontend support (draft PR)