[Xcode] Use the Legacy Build System to use Swift 5 mode on Xcode 10

[ikesyo]

This works around the Swift version validation by the New Build System. See the following for the details:

• https://learningswift.brightdigit.com/swift-5-0-xcode-10-1/
• https://forums.swift.org/t/how-to-set-swift-version-5-for-recent-dev-snapshots-in-xcode-build-settings/18692

Supersedes #1818.

[ikesyo]

@swift-ci Please test

[ikesyo]

This is now set at the project level, not at per-target level.

[parkera]

Hm, do we want to switch to 5.0 mode yet?

[ikesyo]

IIUC it's already built with 5.0 mode on Linux (CMake build).

• Build Foundation tests in Swift 4 mode to work around @autoclosure regression #1774
• Adjust code to adjust to @autoclosure change in Swift 5. #1775

[ikesyo]

11:33:55 + /Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/swift-corelibs-xctest/build_script.py --swiftc=/Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swiftc --build-dir=/Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/xctest-macosx-x86_64 --foundation-build-dir=/Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/foundation-macosx-x86_64 --swift-build-dir=/Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/swift-macosx-x86_64 --release
11:33:56 Build settings from command line:
11:33:56     INDEX_ENABLE_DATA_STORE = NO
11:33:56     OBJROOT = /Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/xctest-macosx-x86_64
11:33:56     SWIFT_EXEC = /Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swiftc
11:33:56     SWIFT_LINK_OBJC_RUNTIME = YES
11:33:56     SYMROOT = /Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/Ninja-ReleaseAssert/xctest-macosx-x86_64
11:33:56     TOOLCHAINS = default
11:33:56 
11:33:57 note: Using new build system

Hm the Xcode project of swift-corelibs-xctest should also be updated or we should have a condition:

#if swift(>=5.0)
    guard let parsingTests = testRoot[kURLTestParsingTestsKey] as? [Any] else {
        XCTFail("Unable to create the parsingTests dictionary")
        return nil
    }
#else
    guard let parsingTests = testRoot?[kURLTestParsingTestsKey] as? [Any] else {
        XCTFail("Unable to create the parsingTests dictionary")
        return nil
    }
#endif

[ikesyo]

Closing this in favor of #1829.