[lldb] 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

lldb/source/Commands/CommandObjectWatchpoint.cpp

@@ -1083,7 +1083,7 @@ protected:
     options.SetUnwindOnError(true);
     options.SetKeepInMemory(false);
     options.SetTryAllThreads(true);
-    options.SetTimeout(llvm::None);
+    options.SetTimeout(std::nullopt);
 
     ExpressionResults expr_result =
         target->EvaluateExpression(expr, frame, valobj_sp, options);