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/tools/clippy/tests/ui/suspicious_arithmetic_impl.rs

@@ -117,7 +117,7 @@ impl Add for Bar {
     type Output = Bar;
 
     fn add(self, other: Self) -> Self {
-        Bar(self.0 & !other.0) // OK: UnNot part of BiExpr as child node
+        Bar(self.0 & !other.0) // OK: Not part of BiExpr as child node
     }
 }
 
@@ -126,7 +126,7 @@ impl Sub for Bar {
 
     fn sub(self, other: Self) -> Self {
         if self.0 <= other.0 {
-            Bar(-(self.0 & other.0)) // OK: UnNeg part of BiExpr as parent node
+            Bar(-(self.0 & other.0)) // OK: Neg part of BiExpr as parent node
         } else {
             Bar(0)
         }