Try to fix libclang reproducer tests after r329465

Try to fix libclang reproducer tests after r329465

They were failing on Windows because the output YAML didn't parse:

  YAML:1:664: error: Unrecognized escape code!

  {"toolchain":"D:\\buildslave\\clang-x64-ninja-win7\\stage1",
    "libclang.operation":"complete", "libclang.opts":1, "args":["clang",
    "-fno-spell-checking",
    "D:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\test\Index\create-libclang-completion-reproducer.c",
    "-Xclang", "-detailed-preprocessing-record",
    "-fallow-editor-placeholders"],
    "invocation-args":["-code-completion-at=D:\buildslave\clang-x64-ninja-win7\llvm\tools\clang\test\Index\create-libclang-completion-reproducer.c:10:1"],
    "unsaved_file_hashes":[{"name":"D:\\buildslave\\clang-x64-ninja-win7\\llvm\\tools\\clang\\test\\Index\\create-libclang-completion-reproducer.c",
      "md5":"aee23773de90e665992b48209351d70e"}]}

This adds some more escaping to try to make it work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@329558 91177308-0d34-0410-b5e6-96231b3b80d8