Merge pull request #60980 from etcwilde/ewilde/stdlib-source-dir

Set stdlib source dir in non stdlib builds

Author: etcwilde

CMakeLists.txt

@@ -1176,6 +1176,7 @@ endif()
 if(SWIFT_BUILD_STDLIB)
   add_subdirectory(stdlib)
 else()
+  set(SWIFT_STDLIB_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/stdlib")
   # Some of the things below depend on the threading library
   add_subdirectory(stdlib/public/Threading)