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.

3rdparty/CMakeLists.txt

@@ -2,6 +2,8 @@ find_package(PkgConfig)
 include(ExternalProject)
 include(CMakeDependentOption)
 
+set(CMAKE_CXX_STANDARD 20)
+
 # Dummy target to use when lib isn't available
 add_library(3rdparty_dummy_lib INTERFACE)