Auto merge of rust-lang#108955 - Nilstrieb:dont-use-me-pls, r=oli-obk

Add `internal_features` lint

Implements rust-lang/compiler-team#596

Also requires some more test blessing for codegen tests etc

`@jyn514` had the idea of just `allow`ing the lint by default in the test suite. I'm not sure whether this is a good idea, but it's definitely one worth considering. Additional input encouraged.

tests/compile-test.rs

@@ -140,6 +140,7 @@ fn base_config(test_dir: &str) -> compiletest::Config {
         [
             "--emit=metadata",
             "-Aunused",
+            "-Ainternal_features",
             "-Zui-testing",
             "-Dwarnings",
             &format!("-Ldependency={}", deps_path.display()),