[ASTImporter] Silence unused variable warning in Release builds. NFC.

llvm-svn: 364774
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index b2de045..7d69ed9 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -8655,6 +8655,7 @@
 
 void ASTImporter::setImportDeclError(Decl *From, ImportError Error) {
   auto InsertRes = ImportDeclErrors.insert({From, Error});
+  (void)InsertRes;
   // Either we set the error for the first time, or we already had set one and
   // now we want to set the same error.
   assert(InsertRes.second || InsertRes.first->second.Error == Error.Error);