[flang] Fix bug with use-associated base of component reference

When the base of a component reference (`a` in `a%b`) was use
associated, we were failing to follow it to the real symbol.

Fixes flang-compiler/f18#616.

Original-commit: flang-compiler/f18@95bfac6f65eccfedd03d791f01267d43497c6153
Reviewed-on: https://github.com/flang-compiler/f18/pull/617