[build] Disallow calls to objc_msgSend() with no function pointer cast.

Author: Greg Parker

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -280,6 +280,9 @@ macro(swift_common_cxx_warnings)
   if("${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4068")
   endif()
+
+  # Disallow calls to objc_msgSend() with no function pointer cast.
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DOBJC_OLD_DISPATCH_PROTOTYPES=0")
 endmacro()
 
 # Like 'llvm_config()', but uses libraries from the selected build