Rollup merge of #81913 - osa1:rename_unop_variants, r=matthewjasper

Rename HIR UnOp variants

This renames the variants in HIR UnOp from

    enum UnOp {
        UnDeref,
        UnNot,
        UnNeg,
    }

to

    enum UnOp {
        Deref,
        Not,
        Neg,
    }

Motivations:

- This is more consistent with the rest of the code base where most enum
  variants don't have a prefix.

- These variants are never used without the `UnOp` prefix so the extra
  `Un` prefix doesn't help with readability. E.g. we don't have any
  `UnDeref`s in the code, we only have `UnOp::UnDeref`.

- MIR `UnOp` type variants don't have a prefix so this is more
  consistent with MIR types.

- "un" prefix reads like "inverse" or "reverse", so as a beginner in
  rustc code base when I see "UnDeref" what comes to my mind is
  something like `&*` instead of just `*`.

compiler/rustc_middle/src/ty/consts.rs

@@ -55,7 +55,7 @@ impl<'tcx> Const<'tcx> {
 
         let lit_input = match expr.kind {
             hir::ExprKind::Lit(ref lit) => Some(LitToConstInput { lit: &lit.node, ty, neg: false }),
-            hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => match expr.kind {
+            hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => match expr.kind {
                 hir::ExprKind::Lit(ref lit) => {
                     Some(LitToConstInput { lit: &lit.node, ty, neg: true })
                 }