swift234

IRGen: Don't lay out associated type witness table slots for protocols that don't need them.

Fix a layout discrepancy when an associated type had @objc protocol constraints, in which case WitnessTableLayout would reserve a slot for the witness table, but WitnessTableBuilder would never emit it. Also add assertions to WitnessTableBuilder that check that the witness table we're building follows the claimed WitnessTableLayout. Fixes rdar://problem/20418117.

Swift SVN r26953