swift234

Code completion: fix use-after-free

A use-after-free could happen in the following scenario:

- code completion caches results from module A;
- code completion returns cached results -- result set 1;
- module is rebuilt;
- another code completion request arrives, cache is invalidated and re-filled.
  Old results are freed, even though someone could be still using the result
  set.

rdar://16953614


Swift SVN r18329