Move lint_store from GlobalCtxt to Session.

This was made possible by the removal of plugin support, which
simplified lint store creation.

This simplifies the places in rustc and rustdoc that call
`describe_lints`, which are early on. The lint store is now built before
those places, so they don't have to create their own lint store for
temporary use, they can just use the main one.

Author: nnethercote

tests/ui/macro_use_imports.stderr

@@ -1,23 +1,23 @@
 error: `macro_use` attributes are no longer needed in the Rust 2018 edition
-  --> $DIR/macro_use_imports.rs:25:5
+  --> $DIR/macro_use_imports.rs:23:5
    |
 LL |     #[macro_use]
-   |     ^^^^^^^^^^^^ help: remove the attribute and import the macro directly, try: `use mac::inner::nested::string_add;`
+   |     ^^^^^^^^^^^^ help: remove the attribute and import the macro directly, try: `use mac::{inner::mut_mut, inner::try_err};`
    |
    = note: `-D clippy::macro-use-imports` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::macro_use_imports)]`
 
 error: `macro_use` attributes are no longer needed in the Rust 2018 edition
-  --> $DIR/macro_use_imports.rs:23:5
+  --> $DIR/macro_use_imports.rs:21:5
    |
 LL |     #[macro_use]
-   |     ^^^^^^^^^^^^ help: remove the attribute and import the macro directly, try: `use mac::{inner::mut_mut, inner::try_err};`
+   |     ^^^^^^^^^^^^ help: remove the attribute and import the macro directly, try: `use mini_mac::ClippyMiniMacroTest;`
 
 error: `macro_use` attributes are no longer needed in the Rust 2018 edition
-  --> $DIR/macro_use_imports.rs:21:5
+  --> $DIR/macro_use_imports.rs:25:5
    |
 LL |     #[macro_use]
-   |     ^^^^^^^^^^^^ help: remove the attribute and import the macro directly, try: `use mini_mac::ClippyMiniMacroTest;`
+   |     ^^^^^^^^^^^^ help: remove the attribute and import the macro directly, try: `use mac::inner::nested::string_add;`
 
 error: `macro_use` attributes are no longer needed in the Rust 2018 edition
   --> $DIR/macro_use_imports.rs:19:5