The name "flag" implies a type of command line option which is no longer
how these variables are used in multilib. Although tags often look
similar to command line options this is not a requirement - tags may be
arbitrary strings.
MultilibBuilder still uses the name "flag" since it is designed to be
used only with valid command line options, even though these are
subsequently used as multilib tags. Similarly addMultilibFlag() keeps
its existing name because of how it is used, even though it now takes a
tag_set argument.
An "option" here seems to be the same thing as an "argument" elsewhere in this paragraph. Since the terminology is already confusing, perhaps simplify by using the same word consistently throughout? I think "option" is more precise, because positional clang arguments like input files definitely don't play a part in this mechanism.