Don't leak when expanding response files.

Before this patch we would strdup each argument. If one was a response file,
we would replace it with the response file contents, leaking the original
strdup result.

We now don't strdup the originals and let StringSaver free any memory it
allocated. This also saves a bit of malloc traffic when response files are
not used.

Leak found by the valgrind build bot.

llvm-svn: 187042