Support: Fix handling of args that begin with @ but aren't files

Command line arguments that begin with @ but aren't a path to an
existing file currently cause later @file arguments to be ignored.

Correctly skip over these arguments instead of trying to read a
non-existent file 20 times and giving up.

Since the problem manifests in the clang driver, the test is in that
repository.

Fixes rdar://problem/15590906