The change to the forward-scanning rule regressed some diagnostics,
because we no longer generated the special "trailing closure mismatch"
diagnostic. Reinstate the special-case "trailing closure mismatch"
diagnostic, but this time do so as part of the normal argument
mismatch diagnostics so it is based on type information.
While here, clean up the handling of missing-argument diagnostics to
deal with (multiple) trailing closures properly, so that we can (e.g)
suggest adding a new labeled trailing closure at the end, rather than
producing nonsensical Fix-Its.
And, note that SR-12291 is broken (again) by the forward-scan matching
rules.
To preserve subtype relationship between element types of a collection
passed as an argument to a parameter represented by another collection
type let's extend opening of the collection types to be done for arguments
to @autoclosure parameters as well because implicit closure is transparent
to the caller.
Resolves: [SR-13135](https://bugs.swift.org/browse/SR-13135)
Resolves: rdar://problem/65088975
