Removed #[rustc_error] from tests that are all // compile-pass.

I also added `// skip-codegen` to each one, to address potential concerns
that this change would otherwise slow down our test suite spending time
generating code for files that are really just meant to be checks of
compiler diagnostics.

(However, I will say: My preference is to not use `// skip-codegen` if
one can avoid it. We can use all the testing of how we drive LLVM that
we can get...)

(Updated post rebase.)

src/test/ui/lint/lint-unknown-feature-default.rs

@@ -14,7 +14,7 @@
 // FIXME(#44232) we should warn that this isn't used.
 #![feature(rust1)]
 
-#![feature(rustc_attrs)]
+// compile-pass
 
-#[rustc_error]
-fn main() { } //~ ERROR: compilation successful
+
+fn main() { }