NLL: errors from free variables should highlight their span, not span of their lambda expression

[pnkfelix]

NLL has regressed some test cases in that it will highlight the whole span of a lambda expression (i.e. the |args, ... | { body ... }) rather than focus in on the occurrence(s) of the relevant free variable(s) within that lambda expression.

One instances of this is:

https://github.com/rust-lang/rust/blob/master/src/test/ui/region-borrow-params-issue-29793-small.nll.stderr

compare that output with that from the old AST borrowck:

https://github.com/rust-lang/rust/blob/master/src/test/ui/region-borrow-params-issue-29793-small.nll.stderr

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(In addition, AST borrowck is noting that such a capture of a free variable is occurring, and highlights the parameter list saying "capture occurs here"; but this additional detail seems like very low priority to fix, if we want to fix it at all.)