Fixes build issue with LLVM on Windows with cmake (#10776)

LLVM fails to build on Windows with C++ 20 standard.  When building LLVM
on Windows, we use C++ 14 already.  This commit changes the c++ standard
to be picked by each subdirectory rather than the standard being picked
by the root level cmake file.

CMakeLists.txt

@@ -26,7 +26,6 @@ option(USE_PRECOMPILED_HEADERS "Use precompiled headers" OFF)
 
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/buildfiles/cmake")
 
-set(CMAKE_CXX_STANDARD 20)
 include(CheckCXXCompilerFlag)
 
 if(NOT CMAKE_BUILD_TYPE)