swift234

[cmake] Move SwiftApplyXcodeSubstitutions.cmake => SwiftXcodeSupport.cmake.

As an additional fix, this moves the include of SwiftXcodeSupport to the one
place where its functionality is used, AddSwift.cmake. Keeping the include in
./CMakeLists.txt makes it seem like the functionality is needed there (when it
is not).

In a subsequent commit, I am going to refactor the xcode support I have been
adding to SwiftSharedCMakeConfig into this file and have SwiftSharedCMakeConfig
import it.