embedded: make sure to generate witness tables which are imported from other modules

In embedded swift all the code is generated in the top-level module.
De-serialized witness tables for class existentials must be code-gen'd and therefore made non-external.

Fixes an unresolved symbol linker error.
rdar://142561676

lib/SIL/IR/Linker.cpp

@@ -324,6 +324,17 @@ void SILLinkerVisitor::visitProtocolConformance(
     return;
   }
 
+  if (Mod.getASTContext().LangOpts.hasFeature(Feature::Embedded) &&
+      isAvailableExternally(WT->getLinkage()) &&
+      WT->getProtocol()->requiresClass()) {
+    // In embedded swift all the code is generated in the top-level module.
+    // De-serialized tables (= public_external) must be code-gen'd and
+    // therefore made non-external.
+    // Note: for functions we do that at the end of the pipeline in the
+    // IRGenPrepare pass to be able to eliminate dead functions.
+    WT->setLinkage(SILLinkage::Hidden);
+  }
+
   auto maybeVisitRelatedConformance = [&](ProtocolConformanceRef c) {
     // Formally all conformances referenced by a used conformance are used.
     // However, eagerly visiting them all at this point leads to a large blowup

test/embedded/modules-class-bound-existential.swift

@@ -0,0 +1,41 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -emit-module -o %t/MyModule.swiftmodule %t/MyModule.swift -enable-experimental-feature Embedded -parse-as-library
+// RUN: %target-swift-frontend -c -I %t %t/Main.swift -enable-experimental-feature Embedded -parse-as-library -o %t/a.o
+// RUN: %target-clang -x c -c %S/Inputs/print.c -o %t/print.o
+// RUN: %target-clang %t/a.o %t/print.o -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx || OS=linux-gnu
+// REQUIRES: swift_feature_Embedded
+
+// BEGIN MyModule.swift
+
+public protocol Foo: AnyObject {
+  func run()
+}
+
+public class Hello: Foo {
+  public init() {}
+  public func run() {
+    print("Hello from MyLibrary!")
+  }
+}
+
+// BEGIN Main.swift
+
+import MyModule
+
+@main
+struct MyApp {
+  static func main() {
+    let ex: Foo = Hello()
+    print("Hello from main!")
+    ex.run()
+  }
+}
+
+// CHECK: Hello from main!