[flang][driver] deprecate manual usage of -lFortran_main (#79016)

Intended to warn users of the 18.x release not to do this.

A better solution should be found for the 19.x release. See discussion
in https://github.com/llvm/llvm-project/pull/78152.

Unfortunately there is no warning on Windows currently. I am rushing to
get this landed before 18.x branches.