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.

compiler/rustc_codegen_ssa/src/lib.rs

@@ -26,7 +26,7 @@
 use rustc_macros::{Decodable, Encodable};
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::dep_graph::WorkProduct;
-use rustc_middle::lint::LevelSpec;
+use rustc_middle::lint::StableLevelSpec;
 use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerFile;
 use rustc_middle::middle::dependency_format::Dependencies;
 use rustc_middle::middle::exported_symbols::SymbolExportKind;
@@ -342,8 +342,8 @@ pub fn deserialize_rlink(
 /// Instead, encode exactly the information we need.
 #[derive(Copy, Clone, Debug, Encodable, Decodable)]
 pub struct CodegenLintLevelSpecs {
-    linker_messages: LevelSpec,
-    linker_info: LevelSpec,
+    linker_messages: StableLevelSpec,
+    linker_info: StableLevelSpec,
 }
 
 impl CodegenLintLevelSpecs {