Return true on success in cl::ExpandResponseFiles

This fixes a logic bug pointed out by Juraj Ivancic.

No behavior change because none of the in-tree clients of
cl::ExpandResponseFiles check the return value.  In this case, the
@prefixed arguments are left in the command line.  Downstream command
line processing has the opportunity to emit errors about it, so this
isn't that bad.

llvm-svn: 196306