[MLIR] Suppress warning from usage of C99 flexible array members on AppleClang. (#4204)

* Remove usage of C99 array.

* Try to fix ABI mismatch.

* Suppress warning on AppleClang.

* Revert back to using C99 flexible array member and suppress warning
instead as suggested by https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

cmake/clang_4_0_flags.cmake

@@ -35,6 +35,7 @@ if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "AppleClang")
     if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 9.1.0)
         #set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-zero-as-null-pointer-constant")
     endif()
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-c99-extensions")
 endif()
 
 # should remove these