swift234

IRGen: Populate vtables using the SILVTable instead of reinventing AST lookup.

We still preserve IRGen's current ordering of vtable slots, but use SIL's record of which SILFunction implements which method instead of walking overrides independently. Another step on the way to rdar://problem/19321484; if SILGen determines that a thunk is needed to interface an override with its base method, IRGen will now pick it up. (Thunk generation is still busted in certain inheritance cases I need to fix before declaring victory though.)

Swift SVN r24732