swift234

Code completion: add a lot of infrastructure code

* Added a mode in swift-ide-test to test code completion.  Unlike c-index-test,
  the code completion token in tests is a real token -- we don't need to
  count lines and columns anymore.

* Added support in lexer to produce a code completion token.

* Added a parser interface to code completion.  It is passed down from the
  libFrontend to the parser, but its functions are not called yet.

* Added a sketch of the interface of code completion consumer and code
  completion results.

Note: all this is not doing anything useful yet.


Swift SVN r6128