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_middle/src/mir/mod.rs

@@ -1213,7 +1213,7 @@ pub enum InlineAsmOperand<'tcx> {
         out_place: Option<Place<'tcx>>,
     },
     Const {
-        value: Operand<'tcx>,
+        value: Box<Constant<'tcx>>,
     },
     SymFn {
         value: Box<Constant<'tcx>>,