Specify rust lints for compiler/ crates via Cargo.

By naming them in `[workspace.lints.rust]` in the top-level
`Cargo.toml`, and then making all `compiler/` crates inherit them with
`[lints] workspace = true`. (I omitted `rustc_codegen_{cranelift,gcc}`,
because they're a bit different.)

The advantages of this over the current approach:
- It uses a standard Cargo feature, rather than special handling in
  bootstrap. So, easier to understand, and less likely to get
  accidentally broken in the future.
- It works for proc macro crates.

It's a shame it doesn't work for rustc-specific lints, as the comments
explain.

compiler/rustc_traits/Cargo.toml

@@ -13,3 +13,6 @@ rustc_span = { path = "../rustc_span" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 tracing = "0.1"
 # tidy-alphabetical-end
+
+[lints]
+workspace = true