Rollup merge of #87916 - nbdd0121:black_box, r=nagisa

Implement `black_box` using intrinsic

Introduce `black_box` intrinsic, as suggested in https://github.com/rust-lang/rust/pull/87590#discussion_r680468700.

This is still codegenned as empty inline assembly for LLVM. For MIR interpretation and cranelift it's treated as identity.

cc `@Amanieu` as this is related to inline assembly
cc `@bjorn3` for rustc_codegen_cranelift changes
cc `@RalfJung` as this affects MIRI

r? `@nagisa` I suppose

compiler/rustc_typeck/src/check/intrinsic.rs

@@ -102,6 +102,7 @@ pub fn intrinsic_operation_unsafety(intrinsic: Symbol) -> hir::Unsafety {
         | sym::maxnumf64
         | sym::type_name
         | sym::forget
+        | sym::black_box
         | sym::variant_count => hir::Unsafety::Normal,
         _ => hir::Unsafety::Unsafe,
     }
@@ -387,6 +388,8 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
                 (1, vec![param_ty; 2], tcx.types.bool)
             }
 
+            sym::black_box => (1, vec![param(0)], param(0)),
+
             other => {
                 tcx.sess.emit_err(UnrecognizedIntrinsicFunction { span: it.span, name: other });
                 return;