test clang++ and gold ld with LTO

test-llvm-gold.sh

echo '#include <string>' > lib.hpp
echo 'std::string hello();' >> lib.hpp
echo '#include "lib.hpp"' > lib.cpp
echo 'std::string hello() { return "hello world\n"; }' >> lib.cpp
echo '#include <iostream>' > test.cpp
echo '#include "lib.hpp"' >> test.cpp
echo 'int main(void) { std::cout << hello();return 0; }' >> test.cpp
clang++ -o test -O4 test.cpp lib.cpp -use-gold-plugin
./test