[Symfony 4] Sub class \Symfony\Component\Validator\ConstraintViolation and use that in \Drupal\Core\TypedData\Validation\ExecutionContext::addViolation()

Problem/Motivation

Symfony 4 uses strict types in the Constraint system, typecasting translateable markup to strings. This causes the twig system to make the string html safe, escaping the html we include in the message.

Proposed resolution

Create a subclassed ConstraintViolation that supports MarkupInterface and use that in core.

Remaining tasks

Create the subclass
Replace usage throughout core
Change Record?

User interface changes

none.

API changes

Switch to the subclassed Violation class for core violations.

Data model changes

none.

Release notes snippet