Try to get at_file_missing.c passing after LLVM r267556.

r267556 made backslashes escape the next character
unconditionally in rsp files.  This test echos a path into
a rsp file, and paths contain backslashes on Windows. Since
it's not important for this test to get the filename from
the rsp file, just pass it regularly.

llvm-svn: 267601