build: handle directories for unified build

The directory paths would collide with a cross-compiled unified build of
LLVM and swift.  Adjust the directories to work in that environment.

Author: compnerd

cmake/modules/SwiftWindowsSupport.cmake

@@ -76,12 +76,12 @@ function(swift_windows_generate_sdk_vfs_overlay flags)
   set(UCRTVersion $ENV{UCRTVersion})
 
   # TODO(compnerd) use a target to avoid re-creating this file all the time
-  configure_file("${CMAKE_SOURCE_DIR}/utils/WindowsSDKVFSOverlay.yaml.in"
-                 "${CMAKE_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
+  configure_file("${SWIFT_SOURCE_DIR}/utils/WindowsSDKVFSOverlay.yaml.in"
+                 "${CMAKE_CURRENT_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
                  @ONLY)
 
   set(${flags}
-      -Xclang;-ivfsoverlay;-Xclang;"${CMAKE_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
+        -Xclang;-ivfsoverlay;-Xclang;"${CMAKE_CURRENT_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
       PARENT_SCOPE)
 endfunction()