[flang] Add check for constraints on event-stmts

In the CoarrayChecker, add checks for the constraints C1177 and
C1178 for event-wait-stmt. Add event-post-stmt to the check
for the constraints for sync-stat-list. Add a check for the
constraint C1176 on event-variable.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D137204