[flang][OpenMP] Fix privatization of threadprivate common block (#77821)

In some cases, when privatizing a threadprivate common block, the
original symbol will correspond to the common block, instead of
its threadprivate version. This can happen, for instance, with a
common block, declared in a separate module, used by a parent
procedure and privatized in its child procedure. In this case,
symbol lookup won't find a symbol in the parent procedure, but
only in the module where the common block was defined.

Fixes https://github.com/llvm/llvm-project/issues/65028