[lldb] Use std::optional instead of llvm::Optional (NFC)

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to clean up the "using" declarations, #include
"llvm/ADT/Optional.h", etc.

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/ScriptInterpreter/Python/ScriptedThreadPythonInterface.h

@@ -30,11 +30,11 @@ public:
 
   lldb::tid_t GetThreadID() override;
 
-  llvm::Optional<std::string> GetName() override;
+  std::optional<std::string> GetName() override;
 
   lldb::StateType GetState() override;
 
-  llvm::Optional<std::string> GetQueue() override;
+  std::optional<std::string> GetQueue() override;
 
   StructuredData::DictionarySP GetStopReason() override;
 
@@ -42,7 +42,7 @@ public:
 
   StructuredData::DictionarySP GetRegisterInfo() override;
 
-  llvm::Optional<std::string> GetRegisterContext() override;
+  std::optional<std::string> GetRegisterContext() override;
 
   StructuredData::ArraySP GetExtendedInfo() override;
 };