Revert "Remove dependence on <ciso646>" (llvm#126399)

Reverts llvm#73273

Author: JDevlieghere

llvm/include/llvm/Support/Threading.h

@@ -19,7 +19,6 @@
 #include "llvm/Config/llvm-config.h" // for LLVM_ON_UNIX
 #include "llvm/Support/Compiler.h"
 #include <optional>
-#include <version>
 
 #if defined(_MSC_VER)
 // MSVC's call_once implementation worked since VS 2015, which is the minimum