swift234

Introduce a new ParseResult<T> helper class, which is effectively
oneof { T*, Absent, ParseError, SemaError }.

This replaces the former convention used by the expression parsing
logic that used NullablePtr + bool to indicate all of these states,
in a way that I could never keep straight.

This should lead to better error recovery, but needs to be adopted by
more parts of the parser.  Sema still uses NullablePtr because it
either returns a valid AST node or has a semantic error, there is no
parse error possible in Sema.



Swift SVN r503