swift234

IRGen: Lower metatypes respecting SIL's 'thin' bit.

Use the 'thin' bit set by SIL to decide whether a metatype lowers to an empty type or not. In GenPoly we still need to accommodate unlowered metatypes to keep protocol witnesses limping along; hopefully that code can be killed soon. With this change we now lower @cc(witness_method) consistently for static methods.

Swift SVN r11535