[flang] Fix use-associated procedure in generic

When a use-associated procedure was included in a generic, we weren't
correctly recording that fact. The ultimate symbol was added rather than
the local symbol.

Also, improve the message emitted for the specific procedure by
mentioning the module it came from.

This fixes one of the problems in https://bugs.llvm.org/show_bug.cgi?id=48648.

Differential Revision: https://reviews.llvm.org/D94696