Unify manual_unwrap_or and manual_unwrap_or_default code

Both lints share a lot of characteristics but were implemented in
unrelated ways. This unifies them, saving around 100 SLOC in the
process, and making one more test trigger the lint. Also, this removes
useless blocks in suggestions.

clippy_utils/src/higher.rs

@@ -176,6 +176,12 @@ impl<'hir> IfLetOrMatch<'hir> {
             ),
         }
     }
+
+    pub fn scrutinee(&self) -> &'hir Expr<'hir> {
+        match self {
+            Self::Match(scrutinee, _, _) | Self::IfLet(scrutinee, _, _, _, _) => scrutinee,
+        }
+    }
 }
 
 /// An `if` or `if let` expression

clippy_utils/src/lib.rs

@@ -1027,6 +1027,7 @@ pub fn is_default_equivalent(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
         ExprKind::Call(from_func, [arg]) => is_default_equivalent_from(cx, from_func, arg),
         ExprKind::Path(qpath) => is_res_lang_ctor(cx, cx.qpath_res(qpath, e.hir_id), OptionNone),
         ExprKind::AddrOf(rustc_hir::BorrowKind::Ref, _, expr) => matches!(expr.kind, ExprKind::Array([])),
+        ExprKind::Block(Block { stmts: [], expr, .. }, _) => expr.is_some_and(|e| is_default_equivalent(cx, e)),
         _ => false,
     }
 }