Treat unstable lints as unknown

This change causes unstable lints to be ignored if the `unknown_lints`
lint is allowed. To achieve this, it also changes lints to apply as soon
as they are processed. Previously, lints in the same set were processed
as a batch and then all simultaneously applied.

Implementation of https://github.com/rust-lang/compiler-team/issues/469

compiler/rustc_span/src/symbol.rs

@@ -1383,6 +1383,7 @@ symbols! {
         test_case,
         test_removed_feature,
         test_runner,
+        test_unstable_lint,
         then_with,
         thread,
         thread_local,