Rollup merge of #83916 - Amanieu:asm_anonconst, r=petrochenkov

Use AnonConst for asm! constants

This replaces the old system which used explicit promotion. See #83169 for more background.

The syntax for `const` operands is still the same as before: `const <expr>`.

Fixes #83169

Because the implementation is heavily based on inline consts, we suffer from the same issues:
- We lose the ability to use expressions derived from generics. See the deleted tests in `src/test/ui/asm/const.rs`.
- We are hitting the same ICEs as inline consts, for example #78174. It is unlikely that we will be able to stabilize this before inline consts are stabilized.

compiler/rustc_hir/src/hir.rs

@@ -2347,7 +2347,7 @@ pub enum InlineAsmOperand<'hir> {
         out_expr: Option<Expr<'hir>>,
     },
     Const {
-        expr: Expr<'hir>,
+        anon_const: AnonConst,
     },
     Sym {
         expr: Expr<'hir>,

compiler/rustc_hir/src/intravisit.rs

@@ -1189,7 +1189,6 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
                 match op {
                     InlineAsmOperand::In { expr, .. }
                     | InlineAsmOperand::InOut { expr, .. }
-                    | InlineAsmOperand::Const { expr, .. }
                     | InlineAsmOperand::Sym { expr, .. } => visitor.visit_expr(expr),
                     InlineAsmOperand::Out { expr, .. } => {
                         if let Some(expr) = expr {
@@ -1202,6 +1201,9 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
                             visitor.visit_expr(out_expr);
                         }
                     }
+                    InlineAsmOperand::Const { anon_const, .. } => {
+                        visitor.visit_anon_const(anon_const)
+                    }
                 }
             }
         }