[flang] Fix bug with USE of USE of generic

When merging use associations into a generic, we weren't handling
the case where the name that was use associated was itself a use
association. This is fixed by following that association to its
ultimate symbol (`useUltimate` in `DoAddUse`).

An example of the bug is `m12d` in `resolve17.f90`. `g` is associated
with `gc` in `m12c` which is associated with `gb` in `m12b`. It was that
last association that we weren't correctly following.

Differential Revision: https://reviews.llvm.org/D93343