Argument-to-Parameter mismatch handles conformance failures
related to arguments, so the logic in `MissingConformanceFailure`
which wasn't entirely correct is now completely obsolete.
Resolves: rdar://problem/56234611
This is a step along the way toward handling backward-compatiblity of UTF8View
slicing and preventing inadvertent creation of String instances that keep
inaccessible memory alive.