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 "*".

src/librustdoc/clean/utils.rs

@@ -316,7 +316,7 @@ crate fn is_literal_expr(cx: &DocContext<'_>, hir_id: hir::HirId) -> bool {
             return true;
         }
 
-        if let hir::ExprKind::Unary(hir::UnOp::UnNeg, expr) = &expr.kind {
+        if let hir::ExprKind::Unary(hir::UnOp::Neg, expr) = &expr.kind {
             if let hir::ExprKind::Lit(_) = &expr.kind {
                 return true;
             }