[flang] Distinguish error/warning cases for bad jumps into constructs

Previously, jumps to labels in constructs from exterior statements
would elicit only a warning.  Upgrade these to errors unless the
branch into the construct would enter into only DO, IF, and SELECT CASE
constructs, whose interiors don't scope variables or have other
set-up/tear-down semantics.  Branches into these "safe" constructs
are still errors if they're nested in an unsafe construct that doesn't
also enclose the exterior branch statement.

Differential Revision: https://reviews.llvm.org/D113310