swift234

Parser: add ParserResult<T> -- a wrapper around the AST node plus extra bits

This allows the parser to recover, create an AST node, return it to the caller
*and* signal the caller that there was an error to trigger recovery in the
caller.  Until now the error was signalled with a nullptr result (any non-null
result was considered a success and no recovery was done in that case).

This also allows us to signal the caller if there was a code completion token
inside the production we tried to parse to trigger delayed parsing in the
caller while doing recovery in the callee.  Until now we could not do recovery
in the callee so that the caller could find the code completion token.

Right now we don't take any advantage of these features.  This commit just
replaces some uses of NullablePtr with ParserResult.


Swift SVN r7332