Re-implemented Path::createDirectoryOnDisk (for Unix).

This method allows one to create a directory, and optionally create all parent
directories that do not exist.

The original implementation would require that *all* directories along a path
are writable by the user, including directories that already exist. For example,
suppose we wanted to create the directory "/tmp/foo/bar", and the directory
"/tmp" already exists, but not "/tmp/foo". Since "/tmp" is writable by all
users, the original implementation would work, and create "/tmp/foo", followed
by "/tmp/bar".

A problem occurred, however if one wanted to created the directory
"/Users/myuser/bar" (or equivalently "/home/myuser/bar"), and "/Users/myuser"
already existed and is writable by the current user. The directory
"/User/myuser" is writable by the user, but "/User" is not. The original
implementation of createDirectoryOnDisk would return with failure since "/User"
is not writable, even though "/User/mysuser" is writable.

The new implementation works by recursively creating parents as needed, and thus
doesn't need to check the permissions on every directory in a path.

llvm-svn: 49162