[flang] Fix semantic analysis for "forall" targeted by "label"

As Fortran 2018 3.18 states, the branch target statement can be
`forall-construct-stmt`, but cannot be `forall-stmt`. `forall-stmt` is
wrapped by `Statement` in `action-stmt` and `action-stmt` can be one
branch target statement. Fix the semantic analysis and add two
regression test cases in lowering.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D123373