[flang] Move ExprChecker into separate pass

DoConcurrentChecker depends on expressions being fully resolved so it
can't be in the same pass as ExprChecker. The same will probably
apply to AssignmentChecker when its finished.

Checks that don't depend on expressions can go in the first pass
with ExprChecker.

Original-commit: flang-compiler/f18@c0785ec06f58098d262cf2f791a98e74bdcef11b
Reviewed-on: https://github.com/flang-compiler/f18/pull/315