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 "*".

clippy_lints/src/floating_point_arithmetic.rs

@@ -129,7 +129,7 @@ fn get_specialized_log_method(cx: &LateContext<'_>, base: &Expr<'_>) -> Option<&
 fn prepare_receiver_sugg<'a>(cx: &LateContext<'_>, mut expr: &'a Expr<'a>) -> Sugg<'a> {
     let mut suggestion = Sugg::hir(cx, expr, "..");
 
-    if let ExprKind::Unary(UnOp::UnNeg, inner_expr) = &expr.kind {
+    if let ExprKind::Unary(UnOp::Neg, inner_expr) = &expr.kind {
         expr = &inner_expr;
     }
 
@@ -541,12 +541,12 @@ fn is_zero(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 /// If the two expressions are not negations of each other, then it
 /// returns None.
 fn are_negated<'a>(cx: &LateContext<'_>, expr1: &'a Expr<'a>, expr2: &'a Expr<'a>) -> Option<(bool, &'a Expr<'a>)> {
-    if let ExprKind::Unary(UnOp::UnNeg, expr1_negated) = &expr1.kind {
+    if let ExprKind::Unary(UnOp::Neg, expr1_negated) = &expr1.kind {
         if eq_expr_value(cx, expr1_negated, expr2) {
             return Some((false, expr2));
         }
     }
-    if let ExprKind::Unary(UnOp::UnNeg, expr2_negated) = &expr2.kind {
+    if let ExprKind::Unary(UnOp::Neg, expr2_negated) = &expr2.kind {
         if eq_expr_value(cx, expr1, expr2_negated) {
             return Some((true, expr1));
         }