[flang] Fix USE with homonymous renaming

Fortran requires that a USE with renaming prevent the USE'd symbol
from also being associated into a scope without renaming.  The
implementation in name resolution gets confused in the case of
a USE with renaming using the same name ("x => x").  Clean things
up.  Fixes LLVM bug https://github.com/llvm/llvm-project/issues/63397.

Differential Revision: https://reviews.llvm.org/D153452