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
