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_ast_lowering/src/expr.rs

@@ -1411,9 +1411,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             out_expr: out_expr.as_ref().map(|expr| self.lower_expr_mut(expr)),
                         }
                     }
-                    InlineAsmOperand::Const { ref expr } => {
-                        hir::InlineAsmOperand::Const { expr: self.lower_expr_mut(expr) }
-                    }
+                    InlineAsmOperand::Const { ref anon_const } => hir::InlineAsmOperand::Const {
+                        anon_const: self.lower_anon_const(anon_const),
+                    },
                     InlineAsmOperand::Sym { ref expr } => {
                         hir::InlineAsmOperand::Sym { expr: self.lower_expr_mut(expr) }
                     }