cmakefile change to require c++14 (#2529)

The latest llvm trunk (llvm 10) built require c++14.
Built with gcc and llvm 10 headers, the following
error will show up:
  ...
  /home/yhs/work/llvm-project/llvm/build/install/include/llvm/Support/RWMutex.h:
      At global scope:
  /home/yhs/work/llvm-project/llvm/build/install/include/llvm/Support/RWMutex.h:101:8:
      error: ‘shared_t$
  med_mutex’ in namespace ‘std’ does not name a type
     std::shared_timed_mutex impl;
  ...
  /home/yhs/work/llvm-project/llvm/build/install/include/llvm/Support/TrailingObjects.h:252:19:
      error: ‘is_final’ is not a member of ‘std’
       static_assert(std::is_final<BaseTy>(), "BaseTy must be final.");
  ...

std::shared_timed_mutex and std::is_final etc. are all c++14 features.
Most compilers should already support c++14. Let us require it in
Makefile now.

Signed-off-by: Yonghong Song <yhs@fb.com>

CMakeLists.txt

@@ -86,7 +86,7 @@ if (USINGISYSTEM AND GCC_VERSION VERSION_LESS 6.0)
 endif()
 
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD 14)
 
 endif(NOT PYTHON_ONLY AND ENABLE_CLANG_JIT)