[flang] Fix bug determining type of statement entity

A statement entity (`data-i-do-variable` or `ac-do-variable`) that
doesn't have a type specified gets the type it would have in the
enclosing scope. That means if there is a visible variable with the
same name, the statement entity gets its type. We were failing to
do that and just applying the implicit rules.

Original-commit: flang-compiler/f18@72bc7c29ba8d4be6a7c93080bb7fac7a3bc73577
Reviewed-on: https://github.com/flang-compiler/f18/pull/305
Tree-same-pre-rewrite: false