Use -Wunused_crate_dependencies for compiler crates.

It's very useful. There are some false positives involving integration
tests in `rustc_pattern_analysis` and `rustc_serialize`. There is also a
false positive involving `rustc_driver_impl`'s
`rustc_randomized_layouts` feature. And I removed a `rustc_span` mention
in a doc comment in `rustc_log` because it wasn't integral to the
comment but caused a dev-dependency.

compiler/rustc_pattern_analysis/tests/intersection.rs

@@ -1,5 +1,7 @@
 //! Test the computation of arm intersections.
 
+#![allow(unused_crate_dependencies)]
+
 use common::*;
 use rustc_pattern_analysis::MatchArm;
 use rustc_pattern_analysis::pat::DeconstructedPat;