NLL: missing suggestion to use `move` keyword on closures

[pnkfelix]

In a number of tests, NLL regresses the diagnostics in that they stop including the suggestion: “to force the closure to take ownership, use the move keyword.”

This is relevant to the following ui/ stderr test files:

• https://github.com/rust-lang/rust/blob/master/src/test/ui/region-borrow-params-issue-29793-small.nll.stderr
• https://github.com/rust-lang/rust/blob/master/src/test/ui/regions-nested-fns-2.nll.stderr
• https://github.com/rust-lang/rust/blob/master/src/test/ui/borrowck/borrowck-escaping-closure-error-1.nll.stderr
• https://github.com/rust-lang/rust/blob/master/src/test/ui/borrowck/borrowck-escaping-closure-error-2.nll.stderr

Update: various diagnostics have improved to the point where I can now identify them as belonging to this issue, when before they were too impoverished for me to tell. I'm adding to the list above as I discover such cases.

[nikomatsakis]

I'll mark this as Edition Preview 2 because it's a .. fairly useful hint? Hard to judge these things.