Fix case when ExprUseVisitor is called after typeck writeback

Fix case when ExprUseVisitor is called after typeck writeback

Clippy uses `ExprUseVisitor` and atleast in some cases it runs
after writeback.

We currently don't writeback the min_capture results of closure
capture analysis since no place within the compiler itself uses it.

In the short term to fix clippy we add a fallback when walking captures
of a closure to check if closure_capture analysis has any entries in it.

Writeback for closure_min_captures will be implemented in
rust-lang/project-rfc-2229#18