[flang] Fix handling of DataRef when analyzing CoindexedNamedObject

As we loop through the Components, maintain a pointer to the current
DataRef rather than moving it. This is more efficient and the previous
behavior caused illegal memory accesses.

Original-commit: flang-compiler/f18@cede2971ff863b8dd6a2c8727f6566a444ac7a52
Reviewed-on: https://github.com/flang-compiler/f18/pull/1050
Tree-same-pre-rewrite: false