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_hir/src/definitions.rs

@@ -409,7 +409,7 @@ impl Definitions {
     }
 
     pub fn expansion_that_defined(&self, id: LocalDefId) -> ExpnId {
-        self.expansions_that_defined.get(&id).copied().unwrap_or(ExpnId::root())
+        self.expansions_that_defined.get(&id).copied().unwrap_or_else(ExpnId::root)
     }
 
     pub fn parent_module_of_macro_def(&self, expn_id: ExpnId) -> DefId {