Constrain maximum lifetime of stack closures that capture variables to
be limited by the innermost repeating scope.

Fixes #7336.
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auto merge of #7455 : nikomatsakis/rust/issue-7336-constrain-closure-lifetimes, r=catamorphism

Constrain maximum lifetime of stack closures that capture variables to be limited by the innermost repeating scope.

Fixes #7336.

r? whomever.