[Concepts] Fix incorrect control flow when TryAnnotateTypeConstraint annotates...

[Concepts] Fix incorrect control flow when TryAnnotateTypeConstraint annotates an invalid template-id

TryAnnotateTypeConstraint could annotate a template-id which doesn't end up being a type-constraint,
in which case control flow would incorrectly flow into ParseImplicitInt.

Reenter the loop in this case.
Enable relevant tests for C++20. This required disabling typo-correction during TryAnnotateTypeConstraint
and changing a test case which is broken due to a separate bug (will be reported and handled separately).