This is a fix for the command option parser.

There was a generic catch-all type for path arguments
called "eArgTypePath," and a specialized version
called "eArgTypeFilename."  It turns out all the
cases where we used eArgTypePath we could have
used Filename or we explicitly meant a directory.

I changed Path to DirectoryName, made it use the
directory completer, and rationalized the uses of
Path.

<rdar://problem/12559915>

llvm-svn: 166533