[NFC] [Clang] [Sema] Implement CXXConstructExpr::getUnusedResultAttr (#152950)

This continues my patch series started as #142541 where multiple kinds
of Expr all use the same getUnusedResultAttrImpl.
The test suite indicates there is no change in behavior happening here.

Author: halbi2

clang/include/clang/AST/ExprCXX.h

@@ -1712,6 +1712,19 @@ class CXXConstructExpr : public Expr {
     CXXConstructExprBits.IsImmediateEscalating = Set;
   }
 
+  /// Returns the WarnUnusedResultAttr that is declared on the callee
+  /// or its return type declaration, together with a NamedDecl that
+  /// refers to the declaration the attribute is attached to.
+  std::pair<const NamedDecl *, const WarnUnusedResultAttr *>
+  getUnusedResultAttr(const ASTContext &Ctx) const {
+    return getUnusedResultAttrImpl(getConstructor(), getType());
+  }
+
+  /// Returns true if this call expression should warn on unused results.
+  bool hasUnusedResultAttr(const ASTContext &Ctx) const {
+    return getUnusedResultAttr(Ctx).second != nullptr;
+  }
+
   SourceLocation getBeginLoc() const LLVM_READONLY;
   SourceLocation getEndLoc() const LLVM_READONLY;
   SourceRange getParenOrBraceRange() const { return ParenOrBraceRange; }

clang/lib/Sema/SemaStmt.cpp

@@ -316,17 +316,10 @@ void DiagnoseUnused(Sema &S, const Expr *E, std::optional<unsigned> DiagID) {
       }
     }
   } else if (const auto *CE = dyn_cast<CXXConstructExpr>(E)) {
-    if (const CXXConstructorDecl *Ctor = CE->getConstructor()) {
-      const NamedDecl *OffendingDecl = nullptr;
-      const auto *A = Ctor->getAttr<WarnUnusedResultAttr>();
-      if (!A) {
-        OffendingDecl = Ctor->getParent();
-        A = OffendingDecl->getAttr<WarnUnusedResultAttr>();
-      }
-      if (DiagnoseNoDiscard(S, OffendingDecl, A, Loc, R1, R2,
-                            /*isCtor=*/true))
-        return;
-    }
+    auto [OffendingDecl, A] = CE->getUnusedResultAttr(S.Context);
+    if (DiagnoseNoDiscard(S, OffendingDecl, A, Loc, R1, R2,
+                          /*isCtor=*/true))
+      return;
   } else if (const auto *ILE = dyn_cast<InitListExpr>(E)) {
     if (const TagDecl *TD = ILE->getType()->getAsTagDecl()) {