swift234

[build-script] XCTest xcodebuild uses SWIFT_EXEC

Because the `xcodebuild` invocation used to compile swift-corelibs-xctest on OS X uses a toolchain that is usually out of date, it often fails (for example, when building the newly migrated Swift 3 code).

Pass the path to the freshly built `swiftc` to `xcodebuild` in order to use the latest Swift compiler.