build: allow building libswift with unified builds

When building with unified builds, `CMAKE_SOURCE_DIR` is not the Swift
repository.  Use the locally defined `SWIFT_SOURCE_DIR` variable to
reference the root of the Swift repository.  Because libswift uses
`project` in the CMakeLists, we cannot use `PROJECT_SOURCE_DIR` here.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -786,7 +786,7 @@ function(add_libswift name)
               "-emit-module-path" "${build_dir}/${module}.swiftmodule"
               "-parse-as-library" ${sources}
               "-wmo" ${libswift_compile_options}
-              "-I" "${CMAKE_SOURCE_DIR}/include/swift"
+              "-I" "${SWIFT_SOURCE_DIR}/include/swift"
               "-I" "${build_dir}"
       COMMENT "Building libswift module ${module}")