[cxx-interop] Restore HOSTTOOLS build for C++ stdlib overlay

This commit should be reverted after `-enable-experimental-cxx-interop` flag lands in a released Xcode.

Author: egorzhdan

stdlib/public/Cxx/CMakeLists.txt

@@ -137,7 +137,7 @@ add_swift_target_library(swiftstd ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVE
     SWIFT_MODULE_DEPENDS_WATCHOS Darwin
 
     SWIFT_COMPILE_FLAGS ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
-    -Xfrontend -enable-experimental-cxx-interop
+    -Xfrontend -enable-cxx-interop
     -Xfrontend -module-interface-preserve-types-as-written
 
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}" -lc++