[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/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp

@@ -655,7 +655,7 @@ GetRegistersAsJSON(NativeThreadProtocol &thread) {
       reg_ctx.GetExpeditedRegisters(ExpeditedRegs::Minimal);
 #endif
   if (expedited_regs.empty())
-    return llvm::None;
+    return std::nullopt;
 
   for (auto &reg_num : expedited_regs) {
     const RegisterInfo *const reg_info_p =
@@ -3658,7 +3658,7 @@ GDBRemoteCommunicationServerLLGS::Handle_qWatchpointSupportInfo(
   auto hw_debug_cap = m_current_process->GetHardwareDebugSupportInfo();
 
   StreamGDBRemote response;
-  if (hw_debug_cap == llvm::None)
+  if (hw_debug_cap == std::nullopt)
     response.Printf("num:0;");
   else
     response.Printf("num:%d;", hw_debug_cap->second);