[CS] Better diagnose conformance failures for EnumElementPatterns
Previously we could end up with a
ContextualMismatch fix and a MissingConformance fix
for different elements of the `matchTypes` disjunction,
leading to an ambiguity. Instead, avoid recording
the ContextualMismatch if we're matching an
existential, and tweak the MissingConformance
failure to have a custom diagnostic for
EnumElementPattern matching.
