[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/Utility/StringExtractorGDBRemote.cpp

@@ -638,7 +638,7 @@ bool StringExtractorGDBRemote::ValidateResponse() const {
     return true; // No validator, so response is valid
 }
 
-llvm::Optional<std::pair<lldb::pid_t, lldb::tid_t>>
+std::optional<std::pair<lldb::pid_t, lldb::tid_t>>
 StringExtractorGDBRemote::GetPidTid(lldb::pid_t default_pid) {
   llvm::StringRef view = llvm::StringRef(m_packet).substr(m_index);
   size_t initial_length = view.size();