[clang] Fix missing check for nullptr in CallExpr::getUnusedResultAttr (llvm#118636)

Fixes llvm#117975, a regression introduced by llvm#112521 due to forgetting
to check for `nullptr` before dereferencing in
`CallExpr::getUnusedResultAttr`.

Author: Mick235711

clang/lib/AST/Expr.cpp

@@ -1618,9 +1618,9 @@ QualType CallExpr::getCallReturnType(const ASTContext &Ctx) const {
 std::pair<const NamedDecl *, const Attr *>
 CallExpr::getUnusedResultAttr(const ASTContext &Ctx) const {
   // If the callee is marked nodiscard, return that attribute
-  const Decl *D = getCalleeDecl();
-  if (const auto *A = D->getAttr<WarnUnusedResultAttr>())
-    return {nullptr, A};
+  if (const Decl *D = getCalleeDecl())
+    if (const auto *A = D->getAttr<WarnUnusedResultAttr>())
+      return {nullptr, A};
 
   // If the return type is a struct, union, or enum that is marked nodiscard,
   // then return the return type attribute.

clang/test/SemaCXX/warn-unused-result.cpp

@@ -355,3 +355,12 @@ void use2() {
   (void)G{"Hello"};
 }
 } // namespace nodiscard_specialization
+
+namespace GH117975 {
+// Test for a regression for ICE in CallExpr::getUnusedResultAttr
+int f() { return 0; }
+void id_print_name() {
+  (int) // expected-warning {{expression result unused}}
+    ((int(*)())f)();
+}
+} // namespace GH117975