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_trait_selection/src/traits/select/mod.rs

@@ -279,7 +279,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// tracking is not enabled, just returns an empty vector.
     pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
         assert!(self.intercrate);
-        self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
+        self.intercrate_ambiguity_causes.take().unwrap_or_default()
     }
 
     pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {