[Clang] Always apply `-frtlib-add-rpath` if requested (#220697)
Summary:
This path used to try to infer if it was needed by using the language
options. The result was that if you tried to use this on a `.o` link job
you would not get the rpath behavior you would expect.
The reason it was this way stems from the fact that this used to be
implicit behavior, so it was being conservative. At some point one of
the maintainers dictated that we cannot add rpath by default, so we
ended up with this, but still being conservative.
If the user passes `-frtlib-add-rpath`, just add the rpath. Flang does
this already.