Fix module build after removing None.h from STLExtras.h

Add declaration for NoneType in BitcodeConvenience.h.

Author: cachemeifyoucan

llvm/include/llvm/Bitcode/BitcodeConvenience.h

@@ -35,6 +35,8 @@
 #include <cstdint>
 
 namespace llvm {
+typedef std::nullopt_t NoneType;
+
 namespace detail {
 /// Convenience base for all kinds of bitcode abbreviation fields.
 ///