swift234

Add a specialization of extend for collections.

The specialization uses += operator for collections, which performs much
better than the += operator for sequences.

The extend benchmark from rdar://problem/21689050 improves by:
- 2500x with -O
- 15x with -O -whole-module-optimization

Unfortunately we still seem to be about 100x slower than the user's
memcpy version of the same benchmark with -O -whole-module-optimization.

Swift SVN r30033