Remove dependence on <ciso646> (llvm#73273)

C++23 removed `<ciso646>` from the standard library. The header is used
in two places: Once in order to pull in standard library macros. Since
this file also includes `<optional>`, that use of `<ciso646>` is
technically redundant, but should probably be left in in case a future
change ever removes the include of `<optional>`. A second use of
`<ciso646>` appears to have been introduced in
da65009, but seems unnecessary (the
file doesn't seem to use anything from that header, and it seems to
build just fine on MSVC here without it). The new `<version>` header
should be supported by all supported implementations. This change
replaces uses of `<ciso646>` with the `<version>` header, or removes
them entirely where unnecessary.

Author: michael-kenzel

llvm/include/llvm/Support/Threading.h

@@ -19,6 +19,7 @@
 #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