Split LintExpectationId

`LintExpectationId` has two variants, `Unstable` and `Stable`. There are
some places where both variants are possible, but there are also places
where only one of `Unstable` or `Stable` is possible.

This commit encodes this into the type system by introducing
new types `UnstableLintExpectationId` and `StableLintExpectationId`. The
variants of `LintExpectationId` now enclose these. This makes it clearer
what values are possible where.

Other things of note:
- `LintLevelsProvider` gets an associated type and some method changes.
- `LintContext` gets an associated type.
- `LevelSpec` is made generic. `UnstableLevelSpec` and `StableLevelSpec`
  typedefs are added.
- The unstable types are now guaranteed by the type system to never be
  stably hashed. Previously this was a runtime check.

src/tools/clippy/clippy_lints/src/duplicate_mod.rs

@@ -2,7 +2,7 @@
 use rustc_ast::ast::{Crate, Inline, Item, ItemKind, ModKind};
 use rustc_errors::MultiSpan;
 use rustc_lint::{EarlyContext, EarlyLintPass, Level, LintContext};
-use rustc_middle::lint::LevelSpec;
+use rustc_middle::lint::UnstableLevelSpec;
 use rustc_session::impl_lint_pass;
 use rustc_span::{FileName, Span};
 use std::collections::BTreeMap;
@@ -51,7 +51,7 @@
 struct Modules {
     local_path: PathBuf,
     spans: Vec<Span>,
-    lint_level_specs: Vec<LevelSpec>,
+    lint_level_specs: Vec<UnstableLevelSpec>,
 }
 
 #[derive(Default)]
@@ -96,7 +96,7 @@ fn check_crate_post(&mut self, cx: &EarlyContext<'_>, _: &Crate) {
                 .zip(lint_level_specs)
                 .filter_map(|(span, level_spec)| {
                     if let Some(id) = level_spec.lint_id() {
-                        cx.fulfill_expectation(id);
+                        cx.fulfill_expectation(id.into());
                     }
 
                     (!matches!(level_spec.level(), Level::Allow | Level::Expect)).then_some(*span)