Rollup merge of #51057 - pnkfelix:issue-51025-make-ui-tests-robust-wrt-nll, r=nikomat...

Rollup merge of #51057 - pnkfelix:issue-51025-make-ui-tests-robust-wrt-nll, r=nikomatsakis

make ui tests robust with respect to NLL

This PR revises the `ui` tests that I could quickly identify that:
 1. previously had successful compilations under non-lexical lifetimes (NLL) because they assumed lexical lifetimes, but
 2. such assumption of lexical lifetimes was actually not necessarily part of the spirit of the original issue/bug we want to witness.

In many cases, this is simply a matter of adding a use of a borrow so that it gets extended long enough to observe a conflict.

(In some cases the revision was more subtle, such as adding a destructor, or revising the order of declaration of some variables.)

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With these test revisions in place, I subsequently updated the expected stderr output under the NLL compiletest mode. So now we should get even more testing of NLL than we were before.

Fix #51025