cmStateSnapshot: Return const reference from GetExecutionListFile

Original Description

Add cmStateSnapshot::GetExecutionListFileRef() and cmMakefile::GetExecutionFilePathRef() to stop copying it unconditionally in cmMakefile::ExpandArguments().

Also, consider to make current cmStateSnapshot::GetExecutionListFileRef() to return const std::string&. I was not sure about it because it returns inner iterator, so all client should be aware that if they store reference to returned value, snapshot shouldn't be modified.

Topic-rename: GetExecutionListFile-by-ref