Create member fixes could do a better job of calculating record types

[FMorschel]

Repro:

void foo(String s) {
  var (:String name) = s.pattern;
}

The above doesn't know what the return type should be. We should look into this to see if we can infer ({String name,}) for the above and some other cases.

I'll eventually look into this one. If someone else wants to do that, please mention it in the comments. Thanks!

CC @bwilkerson @srawlins