[CMake] Set SWIFT_ENABLE_PARSEABLE_MODULE_INTERFACES for standalone overlays

This should be true when building standalone overlays as well.

Author: Harlan Haskins

cmake/modules/StandaloneOverlay.cmake

@@ -19,6 +19,7 @@ set(SWIFT_DARWIN_DEPLOYMENT_VERSION_OSX "10.9" CACHE STRING "")
 set(SWIFT_DARWIN_DEPLOYMENT_VERSION_IOS "7.0" CACHE STRING "")
 set(SWIFT_DARWIN_DEPLOYMENT_VERSION_TVOS "9.0" CACHE STRING "")
 set(SWIFT_DARWIN_DEPLOYMENT_VERSION_WATCHOS "2.0" CACHE STRING "")
+set(SWIFT_ENABLE_PARSEABLE_MODULE_INTERFACES TRUE)
 
 set(SWIFT_SOURCE_DIR "${SWIFT_SOURCE_ROOT}/src/swift" CACHE PATH "")
 set(SWIFT_PATH_TO_LLVM_SOURCE "${SWIFT_SOURCE_ROOT}/src/llvm" CACHE PATH "")