Changed unwrap_or to unwrap_or_else in some places.

The discussion seems to have resolved that this lint is a bit "noisy" in
that applying it in all places would result in a reduction in
readability.

A few of the trivial functions (like `Path::new`) are fine to leave
outside of closures.

The general rule seems to be that anything that is obviously an
allocation (`Box`, `Vec`, `vec![]`) should be in a closure, even if it
is a 0-sized allocation.

compiler/rustc_mir/src/borrow_check/region_infer/mod.rs

@@ -582,7 +582,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             self.check_member_constraints(infcx, &mut errors_buffer);
         }
 
-        let outlives_requirements = outlives_requirements.unwrap_or(vec![]);
+        let outlives_requirements = outlives_requirements.unwrap_or_default();
 
         if outlives_requirements.is_empty() {
             (None, errors_buffer)