[flang] Handle EQUIVALENCE and COMMON in size and offset computations

Objects in common blocks have offsets relative to the start of the
common block, independent of the enclosing scope, so they are processed
first. Add alignment to CommonBlockDetails to record the required
alignment of the common block.

For equivalence sets, each object depends on the one that is forced to
occur first in memory. The rest are recorded in the dependents_ map and
have offsets assigned after the other symbols are done.

Differential Revision: https://reviews.llvm.org/D79347