[test][Driver] Fix Clang :: Driver/cl-response-file.c

Clang :: Driver/cl-response-file.c currently FAILs on Solaris:

  Command Output (stderr):
  --
  /vol/llvm/src/clang/dist/test/Driver/cl-response-file.c:10:11: error: CHECK: expected string not found in input
  // CHECK: "-I" "{{.*}}\\Inputs\\cl-response-file\\" "-D" "FOO=2"
            ^

Looking at the generated response file reveals that this is no surprise:

  /I/vol/llvm/src/clang/dist/test/Driver\Inputs

with no newline at the end.  The echo command used to create it boils down to

  echo 'a\cb'

However, one cannot expect \c to be emitted literally: e.g. bash's builtin
echo has

  \c        suppress further output

I've tried various combinations of builtin echo, /usr/bin/echo, GNU echo if
different, the same for printf, and the backslash unescaped and quoted
(a\cb and a\\cb).  The only combination that worked reliably on Solaris,
Linux, and macOS was

  printf 'a\\cb'

so this is what this patch uses.  Tested on amd64-pc-solaris2.11 and
x86_64-pc-linux-gnu.

Differential Revision: https://reviews.llvm.org/D63600

llvm-svn: 363985