Build Swift-Testing with WMO

[etcwilde]

This tells build-script to build Swift-testing with WMO. This results in a faster build products, but is also necessary for configurations using the legacy swift driver, which would emit an invalid swift interface in non-WMO builds.

/home/build-user/build/bootstrap_stage2/swifttesting-linux-x86_64/swift/Testing.swiftinterface:413:2: error: '@_optimize(none)' attribute cannot be applied to stored properties
 411 | }
 412 | @usableFromInline
 413 | @_optimize(none) nonisolated(unsafe) internal var failureBreakpointValue: Swift.Int
     |  `- error: '@_optimize(none)' attribute cannot be applied to stored properties
 414 | @usableFromInline
 415 | @inline(never) internal func failureBreakpoint()

Fixes: rdar://151357567

[etcwilde]

@swift-ci please smoke test

[stmontgomery]

Thanks!