IRGen: Fix TypeInfo for witness_method SIL values.

@convention(witness_method) values were changed to carry a pointer to their source witness table, but the type info wasn't changed to match. Fixing this fixes rdar://problem/26268544.

lib/IRGen/IRGenModule.cpp

@@ -274,6 +274,10 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
     FunctionPtrTy,
     RefCountedPtrTy,
   });
+  WitnessFunctionPairTy = createStructType(*this, "swift.function", {
+    FunctionPtrTy,
+    WitnessTablePtrTy,
+  });
   
   OpaquePtrTy = llvm::StructType::create(LLVMContext, "swift.opaque")
                   ->getPointerTo(DefaultAS);