[CMake] Limit copy-legacy-layouts dependency to the stdlib (#22593)

When we do standalone overlay builds, they should rely on these
layouts already being in the toolchain. (In particular, the target
is never created.)

No tests because this configuration isn't (currently) being tested
outside of Apple.

Author: jrose-apple

cmake/modules/SwiftSource.cmake

@@ -440,9 +440,13 @@ function(_compile_swift_files
   #
   # See stdlib/CMakeLists.txt and TypeConverter::TypeConverter() in
   # lib/IRGen/GenType.cpp.
-  set(SWIFTFILE_PLATFORM "${SWIFT_SDK_${SWIFTFILE_SDK}_LIB_SUBDIR}")
-  set(copy_legacy_layouts_dep
-      "copy-legacy-layouts-${SWIFTFILE_PLATFORM}-${SWIFTFILE_ARCHITECTURE}")
+  if(SWIFTFILE_IS_STDLIB_CORE)
+    set(SWIFTFILE_PLATFORM "${SWIFT_SDK_${SWIFTFILE_SDK}_LIB_SUBDIR}")
+    set(copy_legacy_layouts_dep
+        "copy-legacy-layouts-${SWIFTFILE_PLATFORM}-${SWIFTFILE_ARCHITECTURE}")
+  else()
+    set(copy_legacy_layouts_dep)
+  endif()
 
   add_custom_command_target(
       dependency_target