clang/test/lit.cfg: Tweak getClangBuiltinIncludeDir() not to expose dosish...

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

llvm-svn: 184930