[flang] Consolidate & clean up COMMON block checks (#161286)

COMMON block checks are split between name resolution and declaration
checking. We generally want declaration checks to take place after name
resolution, and the COMMON block checks that are currently in name
resolution have some derived type analyses that are redundant with the
derived type component iteration framework used elsewhere in semantics.
So move as much as possible into declaration checking, use the component
iteration framework, and cope with the missing COMMON block name case
that arises with blank COMMON when placing the error messages.

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