[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/Utility/StringExtractorGDBRemote.cpp

@@ -652,7 +652,7 @@ StringExtractorGDBRemote::GetPidTid(lldb::pid_t default_pid) {
     } else if (view.consumeInteger(16, pid) || pid == 0) {
       // not a valid hex integer OR unsupported pid 0
       m_index = UINT64_MAX;
-      return llvm::None;
+      return std::nullopt;
     }
 
     // "." must follow if we expect TID too; otherwise, we assume -1
@@ -671,7 +671,7 @@ StringExtractorGDBRemote::GetPidTid(lldb::pid_t default_pid) {
   } else if (view.consumeInteger(16, tid) || tid == 0 || pid == AllProcesses) {
     // not a valid hex integer OR tid 0 OR pid -1 + a specific tid
     m_index = UINT64_MAX;
-    return llvm::None;
+    return std::nullopt;
   }
 
   // update m_index