[flang] Semantic checks for critical construct

The commit includes the following,
-> The name field in DoConcurrent*Enforce classes are not used anymore.
Removing the field and its collection and retrieval from
DoConcurrentBodyEnforce and its usage in DoConcurrentLabelEnforce.
-> DoConcurrentLabelEnforce is useful for checking that there
are no branches escaping from other constructs also. For enabling
use in other constructs (like critical) moving this to tools.h
and renaming it as LabelEnforce.
-> Checks for the constraints.
-> Tests for the constaints.

Original-commit: flang-compiler/f18@4b7a007ff3d4a3e519c85e960fc262bb382f3af4
Reviewed-on: https://github.com/flang-compiler/f18/pull/897