Change type guards error message.

[YuichiNukiyama]

Change type guards error message. And add test.
This pull request relate to Error message for type guards are vague about rules #6540 issue.

[msftclas]

Hi @YuichiNukiyama, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!
You've already signed the contribution license agreement. Thanks!

The agreement was validated by Microsoft and real humans are currently evaluating your PR.

TTYL, MSBOT;

[DanielRosenwasser]

Hi @YuichiNukiyama,. rather than modifying the current error message, you should be add a new diagnostic message entirely. What you've done here completely changes all error messages for assignability.

First, you should restore the message to its original contents. Then, when checking the assignability of the guarded type, you should pass that diagnostic message to as the head message to checkTypeAssignableTo.

You'll also need to actually run the tests and accept our baselines. See details in our CONTRIBUTING.md file.