swift234

Improve performance of solving over apply expressions by directly applying the return type whenever possible. This has some nice side-effects:
- Addresses many common user-reported "expression too complex" bugs, including rdar://problem/18876786.
- Shaves up to 10% off of the total time to run our unit tests. (Unscientifically measured on my iMac: 427.46s before, 385.17s after.)

Swift SVN r24514