[Concepts] Recover properly from a RecoveryExpr in a concept

[Concepts] Recover properly from a RecoveryExpr in a concept

Discovered by reducing a different problem, we currently assert because
we failed to make the constraint expressions not dependent, since a
RecoveryExpr cannot be transformed.

This patch fixes that, and gets reasonably nice diagnostics by
introducing a concept (hah!) of "ContainsErrors" to the Satisfaction
types, which causes us to treat the candidate as non-viable.

However, just making THAT candidate non-viable would result in choosing
the 'next best' canddiate, which can result in awkward errors, where we
start evaluating a candidate that is not intended to be selected.
Because of this, and to make diagnostics more relevant, we now just
cause the entire lookup to result in a 'no-viable-candidates'.

This means we will only emit the list of candidates, rather than any
cascading failures.