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.

src/bin/cg_clif.rs

@@ -27,8 +27,8 @@ impl rustc_driver::Callbacks for CraneliftPassesCallbacks {
         config.opts.cg.panic = Some(PanicStrategy::Abort);
         config.opts.debugging_opts.panic_abort_tests = true;
         config.opts.maybe_sysroot = Some(
-            config.opts.maybe_sysroot.clone().unwrap_or(
-                std::env::current_exe()
+            config.opts.maybe_sysroot.clone().unwrap_or_else(
+                || std::env::current_exe()
                     .unwrap()
                     .parent()
                     .unwrap()