[Bitcode] Use std::nullopt instead of None (NFC)

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -528,7 +528,7 @@ private:
   Optional<unsigned> getValueId(GlobalValue::GUID ValGUID) {
     auto VMI = GUIDToValueIdMap.find(ValGUID);
     if (VMI == GUIDToValueIdMap.end())
-      return None;
+      return std::nullopt;
     return VMI->second;
   }
 
@@ -4431,7 +4431,7 @@ void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
 
     auto GetValueId = [&](const ValueInfo &VI) -> Optional<unsigned> {
       if (!VI)
-        return None;
+        return std::nullopt;
       return getValueId(VI.getGUID());
     };