[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/API/SBThread.cpp

@@ -837,7 +837,7 @@ SBError SBThread::StepOverUntil(lldb::SBFrame &sb_frame,
     const bool stop_other_threads = false;
     // TODO: Handle SourceLocationSpec column information
     SourceLocationSpec location_spec(
-        step_file_spec, line, /*column=*/llvm::None, /*check_inlines=*/true,
+        step_file_spec, line, /*column=*/std::nullopt, /*check_inlines=*/true,
         /*exact_match=*/false);
 
     SymbolContextList sc_list;