swift234

Add a basic "re-mangler", which turns a parsed
demangling tree back into a mangled string.

Also, extend the demangling API in a few obvious
ways, and simplify testing for failure in the
node-returning APIs by having them simply return
null instead of a spurious Failure node.

Also, preserve slightly more information in the
demangling tree.  The goal here is eventually to
always allow a perfect round-trip through the
demangler parse tree.  This patch gets us close,
but we're not quite there yet.

Tests to follow.

Swift SVN r24473