Fix missing quotes in CMake for SWIFT_STDLIB_DISABLE_INSTANTIATION_CACHES

Author: kubamracek

stdlib/cmake/modules/SwiftSource.cmake

@@ -527,7 +527,7 @@ function(_compile_swift_files
   endif()
 
   if(SWIFT_STDLIB_DISABLE_INSTANTIATION_CACHES)
-    list(APPEND swift_flags "-Xfrontend -disable-preallocated-instantiation-caches")
+    list(APPEND swift_flags "-Xfrontend" "-disable-preallocated-instantiation-caches")
   endif()
 
   list(APPEND swift_flags ${SWIFT_STDLIB_EXTRA_SWIFT_COMPILE_FLAGS})