Implement is_error_code_v and is_error_condition_v for c++17. Rework the tests...

Implement is_error_code_v and is_error_condition_v for c++17. Rework the tests for is_error_code and is_error_condition, since they were really lacking. Thanks to Alisdair for the heads-up that we were missing these.

llvm-svn: 282331