[ClangImporter] Handle pair to actual struct change

Use `FeatureName` rather than `first`.

Author: bnbarham

lib/ClangImporter/ClangImporter.cpp

@@ -7894,7 +7894,7 @@ bool importer::requiresCPlusPlus(const clang::Module *module) {
   }
 
   return llvm::any_of(module->Requirements, [](clang::Module::Requirement req) {
-    return req.first == "cplusplus";
+    return req.FeatureName == "cplusplus";
   });
 }