This will make sure that compiler developers are using the new driver when they build the compiler locally and use it.
- Adds a new build-script product category: before_build_script_impl for products we wish to build before the impl products.
- Adds a new EarlySwiftDriver product to that category, which gets built with the host toolchain.
- Adds an escape hatch: --skip-early-swift-driver
- Adjusts the swift CMake configuration with an additional step: swift_create_early_driver_symlinks which (if one was built) creates symlinks in the swift build bin directory to the EarlySwiftDriver swift-driver and swift-help executables.
- Adds a new test subset : only_early_swiftdriver, which will get built into a corresponding CMake test target: check-swift-only_early_swiftdriver-* which runs a small subset of driver-related tests against the Early SwiftDriver.
- This subset is run always when the compiler itself is tested (--test is specified)
- With an escape disable-switch: --skip-test-early-swift-driver
- All tests outside of only_early_swiftdriver are forced to run using the legacy C++ driver (to ensure it gets tested, still).
NOTE: SwiftDriver product (no 'Early') is still the main product used to build the driver for toolchain installation into and for executing the product's own tests. This change does not affect that.
