Fix double curlies

Pointed out by Arthur in D57624.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353140 91177308-0d34-0410-b5e6-96231b3b80d8

Author: jfbastien

utils/generate_feature_test_macro_components.py

@@ -865,7 +865,7 @@ def produce_tests():
 
 #endif // TEST_STD_VER > 17
 
-int main(int, char**) { return 0; }
+int main(int, char**) {{ return 0; }}
 """.format(script_name=script_name,
            header=h,
            test_tags=test_tags,