swift234

[build-script] Build Foundation before XCTest

https://github.com/apple/swift-corelibs-xctest/pull/43 introduces
a dependency between XCTest and Foundation. Modify the build script
in order to properly build all products:

- Build Foundation before XCTest, then link Foundation when building
  XCTest by using new '--foundation-build-dir' option.
- Link Foundation when testing SwiftPM by using new '--foundation'
  option.
- On Linux, ensure Foundation is built when XCTest is.