Refactor: Move the mutable parts out of LintStore. Fix #42007.

* #42007 happens because the Session LintStore is emptied when linting.
* The Session LintStore is emptied because the checker (Early/LateContext)
  wants ownership.
* The checker wants ownership because it wants to mutate the pass objects
  and lint levels.

The ownership of the whole store is not essential, only the lint levels and
pass objects need to be owned. Therefore, these parts are extracted out of
the LintStore into a separate structure `LintSession`. The "check crates"
methods can operate on `&mut LintSession` instead of `&mut LintStore`.

This is a minor BREAKING CHANGE for lint writers since the `LintContext`
trait is changed: the `mut_lints` and `level_stack` methods are removed.
But no one outside of `librustc/lint/context.rs` is using these functions,
so it should be safe.

src/test/run-pass/auxiliary/issue_42007_s.rs

@@ -0,0 +1,14 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[repr(u8)]
+pub enum E {
+    B = 1 as u8,
+}

src/test/run-pass/issue-42007.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:issue_42007_s.rs
+
+extern crate issue_42007_s;
+
+enum I {
+    E(issue_42007_s::E),
+}
+
+fn main() {}