[Flang] Set constructExit for Where and Forall constructs

Evaluations for the Where and Forall constructs previously did
not have their constructExit field fixed up. This could lead to
falling through to subsequent case blocks in select case
statements if either a Where or Forall construct was the final part
of one case block. Setting the constructExit field results in the
proper branching behavior.

Fixes issue: https://github.com/llvm/llvm-project/issues/56500

Differential Revision: https://reviews.llvm.org/D129879

Change-Id: Ia868df12084520a935f087524e118bcdf47f6d7a