Make llvm::StringRef to std::string conversions explicit.

This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.

This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.

This doesn't actually modify StringRef yet, I'll do that in a follow-up.
diff --git a/lldb/source/Breakpoint/BreakpointIDList.cpp b/lldb/source/Breakpoint/BreakpointIDList.cpp
index f297b3f..705bc5e 100644
--- a/lldb/source/Breakpoint/BreakpointIDList.cpp
+++ b/lldb/source/Breakpoint/BreakpointIDList.cpp
@@ -144,7 +144,7 @@
         result.SetStatus(eReturnStatusFailed);
         return;
       } else
-        names_found.insert(current_arg);
+        names_found.insert(std::string(current_arg));
     } else if ((i + 2 < old_args.size()) &&
                BreakpointID::IsRangeIdentifier(old_args[i + 1].ref()) &&
                BreakpointID::IsValidIDExpression(current_arg) &&