swift234

SIL: Tweak protocol_method for the SIL-witness-tables regime.

I'd like to treat protocol_method equivalently to archetype_method, but we don't have a way to "open" the implicit type variable inside the existential, so protocol_method still needs to produce a "thick" witness_method reference with the Self polymorphic binding partially applied. We can at least simplify the SIL model by saying that its result is always thick, and let the lowering of @cc(witness_method) @thick in IRGen work out how thick that actually has to be for the given function type, instead of reflecting all the special cases in SIL.

Swift SVN r11330