[flang] Fix bogus error w/ COMMON & EQUIVALENCE (#66254)

Semantic checking of COMMON blocks and EQUIVALENCE sets has an
assumption that the base storage sequence object of each COMMON block
object will also be in that COMMON block's list of objects, and emits an
error message when this is not the case. This assumption is faulty; it
is possible for a base object to have its COMMON block set during offset
assignment.

Fixes https://github.com/llvm/llvm-project/issues/65922.