clang/test/lit.cfg: Tweak getClangBuiltinIncludeDir() not to expose dosish path separator on MSYS bash.exe, since r184774.

    $ bin/clang.exe -print-file-name=include
    e:/path/to/build/bin\..\lib\clang\3.4\include

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184930 91177308-0d34-0410-b5e6-96231b3b80d8