Separate most of rustc::lint::builtin into a separate crate.

This pulls out the implementations of most built-in lints into a
separate crate, to reduce edit-compile-test iteration times with
librustc_lint and increase parallelism. This should enable lints to be
refactored, added and deleted much more easily as it slashes the
edit-compile cycle to get a minimal working compiler to test with (`make
rustc-stage1`) from

    librustc -> librustc_typeck -> ... -> librustc_driver ->
        libcore -> ... -> libstd

to

    librustc_lint -> librustc_driver -> libcore -> ... libstd

which is significantly faster, mainly due to avoiding the librustc build
itself.

The intention would be to move as much as possible of the infrastructure
into the crate too, but the plumbing is deeply intertwined with librustc
itself at the moment. Also, there are lints for which diagnostics are
registered directly in the compiler code, not in their own crate
traversal, and their definitions have to remain in librustc.

This is a [breaking-change] for direct users of the compiler APIs:
callers of `rustc::session::build_session` or
`rustc::session::build_session_` need to manually call
`rustc_lint::register_builtins` on their return value.

This should make #22206 easier.

src/librustdoc/lib.rs

@@ -42,6 +42,7 @@ extern crate rustc;
 extern crate rustc_trans;
 extern crate rustc_driver;
 extern crate rustc_resolve;
+extern crate rustc_lint;
 extern crate serialize;
 extern crate syntax;
 extern crate "test" as testing;