[ELF] - Add test to check we produce an error if unable to write an empty index file.

Test case ensures lld generates an error if unable to
write an empty index file for lazy object file that is not added to link.

This covers the following line with a test:
https://github.com/llvm-mirror/lld/blob/master/ELF/LTO.cpp#L206

llvm-svn: 336340