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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196620
91177308-0d34-0410-b5e6-
96231b3b80d8