cmake: update SwiftSupport from XCTest
cmake: update SwiftSupport from XCTest

Update the swift support CMake rules from XCTest.  This adds support for
additional features like partial module compilation which improves
incremental builds.  It allows for executable and library builds.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch