[flang] When computing the stack of scopes, a label on the first statement

that begins a construct should be considered as in the parent scope
rather that within the scope of the construct that is being entered.

Original-commit: flang-compiler/f18@93da3538bcd31b165535dadefe4d67ee81a42ad9
Reviewed-on: https://github.com/flang-compiler/f18/pull/353