Add scenarios for testing context-tracing
* lib/adlint/cc1/trace.rb : Fix bad context message emission of
  undefinable variables.

* features/code_check/W0460.feature : Add scenario to reproduce bad
  context message problem.
* lib/adlint/exam/c_builtin/cc1_check.rb : Fix bad W0570 code check
  degraded during supporting context-tracing.

* features/code_check/W0570.feature : Add scenario to reproduce a known
  bug.
* lib/adlint/cc1/interp.rb : Fix bad context-tracing during or after
  evaluation of an iteration statement.
* lib/adlint/cc1/object.rb : Ditto.
* lib/adlint/cc1/value.rb : Ditto.

* lib/adlint/cc1/trace.rb : Revise context-tracing interface in order
  not to output context messages about futural controlling-expressions.
* lib/adlint/exam/c_builtin/cc1_check.rb : Ditto.
Merge branch 'feature-dfa_context_mesg' into develop