[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/ScriptedProcessPythonInterface.h

@@ -37,7 +37,7 @@ public:
 
   Status Stop() override;
 
-  llvm::Optional<MemoryRegionInfo>
+  std::optional<MemoryRegionInfo>
   GetMemoryRegionContainingAddress(lldb::addr_t address,
                                    Status &error) override;
 
@@ -56,7 +56,7 @@ public:
 
   bool IsAlive() override;
 
-  llvm::Optional<std::string> GetScriptedThreadPluginName() override;
+  std::optional<std::string> GetScriptedThreadPluginName() override;
 
   StructuredData::DictionarySP GetMetadata() override;