Enable CMake policy CMP0057, which allows `if()` statements to use the `IN_LIST`
operator. In addition, simplify several `if()` statements that used the
`list(FIND ...)` operation instead.
Instead of overwriting all of the paths for the other build configurations with
the one that LLVM selected, I was resetting the path for LLVM's build
configuration over and over again. = /.
This fixes the issue.
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.
