swift234

Refactor how multiple parsing passes and delayed parsing works.

-Introduce PersistentParserState to represent state persistent among multiple parsing passes.
  The advantage is that PersistentParserState is independent of a particular Parser or Lexer object.
-Use PersistentParserState to keep information about delayed function body parsing and eliminate parser-specific
  state from the AST (ParserTokenRange).
-Introduce DelayedParsingCallbacks to abstract out of the parser the logic about which functions should be delayed
  or skipped.

Many thanks to Dmitri for his valuable feedback!

Swift SVN r6580