cpplib: Preliminary implementation of UCNs

[Neil Booth]

Zack Weinberg wrote:-

> I don't want support for UCNs in identifiers to be enabled until we
> have both consensus on how it should work, and a complete
> implementation.  This isn't to say that your code shouldn't go in;
> only that it should be disabled for identifiers, requiring one to edit
> the source code to turn it back on.  That way it's there for
> experimentation, but no one comes to rely on any behavior that we
> are likely to change later.

I agree with this.  Accordingly I've applied the patch with the
following change:

+  /* Is this a syntactically valid UCN?  */
+  if (0 && *buffer->cur == '\\'
+      && (buffer->cur[1] == 'u' || buffer->cur[1] == 'U'))

I'll apply the new test ucn-1.c when I figure out how to properly XFAIL
it; my initial attempts didn't work.

Neil.