[Typechecker] @dynamicMemberLookup can't find subscript without arg label

[theblixguy]

If we don't have a subscript with an argument label, then @dynamicMemberLookup will complain about not finding a subscript(dynamicMember:). For example:

// error: @dynamicMemberLookup attribute requires 'S' to have a 
//        'subscript(dynamicMember:)' method that accepts either 
//        'ExpressibleByStringLiteral' or a keypath
@dynamicMemberLookup 
struct S {
  subscript(dynamicMember: String) -> String {
    fatalError()
  }
}

This can be improved by emitting another diagnostic to tell the user that they need to add an explicit argument label to satisfy the @dynamicMemberLookup requirement.

Resolves SR-10557.

[theblixguy]

Let's see if this works:

@swift-ci please smoke test

[theblixguy]

Hm, I can't seem to run the CI. I think my GitHub ID hasn't been added to the approved users list yet. cc @shahmishal

[xedin]

LGTM! Thank you!

[xedin]

@swift-ci please smoke test

[theblixguy]

It seems like the OSX test is stuck on the last LLDB suite test...

[xedin]

@swift-ci please smoke test macOS platform