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_mir_build/src/thir/cx/expr.rs

@@ -299,7 +299,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
             }
         }
 
-        hir::ExprKind::Unary(hir::UnOp::UnDeref, ref arg) => {
+        hir::ExprKind::Unary(hir::UnOp::Deref, ref arg) => {
             if cx.typeck_results().is_method_call(expr) {
                 overloaded_place(cx, expr, expr_ty, None, vec![arg.to_ref()], expr.span)
             } else {
@@ -307,7 +307,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
             }
         }
 
-        hir::ExprKind::Unary(hir::UnOp::UnNot, ref arg) => {
+        hir::ExprKind::Unary(hir::UnOp::Not, ref arg) => {
             if cx.typeck_results().is_method_call(expr) {
                 overloaded_operator(cx, expr, vec![arg.to_ref()])
             } else {
@@ -315,7 +315,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
             }
         }
 
-        hir::ExprKind::Unary(hir::UnOp::UnNeg, ref arg) => {
+        hir::ExprKind::Unary(hir::UnOp::Neg, ref arg) => {
             if cx.typeck_results().is_method_call(expr) {
                 overloaded_operator(cx, expr, vec![arg.to_ref()])
             } else if let hir::ExprKind::Lit(ref lit) = arg.kind {

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -866,7 +866,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                     return *self.const_to_pat(value, expr.hir_id, expr.span, false).kind;
                 }
                 hir::ExprKind::Lit(ref lit) => (lit, false),
-                hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => {
+                hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => {
                     let lit = match expr.kind {
                         hir::ExprKind::Lit(ref lit) => lit,
                         _ => span_bug!(expr.span, "not a literal: {:?}", expr),