Revert SE-0283 (#34492)

Reverted despite build failures.

lib/IRGen/IRGenModule.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -982,11 +982,6 @@ bool IRGenerator::canEmitWitnessTableLazily(SILWitnessTable *wt) {
   if (wt->getLinkage() == SILLinkage::Shared)
     return true;
 
-  // If we happen to see a builtin witness table here, we can't emit those.
-  // The runtime has those for us.
-  if (isa<BuiltinProtocolConformance>(wt->getConformance()))
-    return false;
-
   NominalTypeDecl *ConformingTy =
     wt->getConformingType()->getNominalOrBoundGenericNominal();