Rollup merge of #86517 - camsteffen:unused-unsafe-async, r=LeSeulArtichaut

Fix `unused_unsafe` around `await`

Enables `unused_unsafe` lint for `unsafe { future.await }`.

The existing test for this is `unsafe { println!() }`, so I assume that `println!` used to contain compiler-generated unsafe but this is no longer true, and so the existing test is broken. I replaced the test with `unsafe { ...await }`. I believe `await` is currently the only instance of compiler-generated unsafe.

Reverts some parts of #85421, but the issue predates that PR.

compiler/rustc_mir/src/transform/check_unsafety.rs

@@ -321,6 +321,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
                 }
                 false
             }
+            Safety::BuiltinUnsafe => true,
             Safety::ExplicitUnsafe(hir_id) => {
                 // mark unsafe block as used if there are any unsafe operations inside
                 if !violations.is_empty() {