Fix #4756: Comments lost when an Existence has grandchildren

[GeoffreyBooth]

Fixes #4756, where comments where lost before return @foo ? 42. This is because the “salvage comments” code in the Existence class was using eachChild, which only looks at the immediate children of the expression, rather than traverseChildren, which drills down to all descendant children. When the expression contains @, as in @foo instead of just foo, the comments are one level deeper than where they would be on just foo.

Previously I was covering accessors via a second check for child.name, but that just masked the real issue (that I wasn’t drilling down far enough) by selectively diving down just in the case of the name property. Now that we’re searching the entire tree, there’s no need for a special check for a name grandchild node.