[flang] Merge use-associated generics

When the same generic is use-associated from two different modules,
they must be merged together into a symbol with GenericDetails.

After that merger, if there is a use association of the same name
with a non-generic we have to report an error. So save the UseDetails
from the original USE in GenericDetails so we can create the
appropriate UseErrorDetails.

Fixes flang-compiler/f18#586.

Original-commit: flang-compiler/f18@5067345f70af1fa262803535515dc644828b43ae
Reviewed-on: https://github.com/flang-compiler/f18/pull/591