Turn copies into references as suggested by clang-tidy's performance-unnecessary-copy-initialization.

llvm-svn: 270994
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 1eac1d2..39f78c3 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -6541,7 +6541,7 @@
     }
   }
 
-  const std::string customGCCName = D.getCCCGenericGCCName();
+  const std::string &customGCCName = D.getCCCGenericGCCName();
   const char *GCCName;
   if (!customGCCName.empty())
     GCCName = customGCCName.c_str();