SIL: Fix calls of @_weakLinked functions from serialized SIL

We have to set the SILFunction's 'weakLinked' flag in the defining
module too, so that it can be serialized. Otherwise when we
deserialize it we are not weak linking calls to the function.

Author: slavapestov

lib/AST/Decl.cpp

@@ -548,7 +548,8 @@ bool Decl::isWeakImported(ModuleDecl *fromModule,
     return false;
 
   auto containingContext =
-      AvailabilityInference::availableRange(this, fromModule->getASTContext());
+      AvailabilityInference::availableRange(this,
+                                            containingModule->getASTContext());
   if (!fromContext.isContainedIn(containingContext))
     return true;
 

lib/SIL/SILFunctionBuilder.cpp

@@ -155,7 +155,7 @@ SILFunctionBuilder::getOrCreateFunction(SILLocation loc, SILDeclRef constant,
     if (constant.isForeign && decl->hasClangNode())
       F->setClangNodeOwner(decl);
 
-    if (decl->isWeakImported(mod.getSwiftModule(), availCtx))
+    if (decl->isWeakImported(/*forModule=*/nullptr, availCtx))
       F->setWeakLinked();
 
     if (auto *accessor = dyn_cast<AccessorDecl>(decl)) {

validation-test/Evolution/Inputs/backward_deploy_always_emit_into_client.swift

@@ -0,0 +1,20 @@
+public func getVersion() -> Int {
+#if BEFORE
+  return 0
+#else
+  return 1
+#endif
+}
+
+#if AFTER
+@_weakLinked @usableFromInline func weakFunction() -> String {
+  return "new"
+}
+
+@_alwaysEmitIntoClient public func serializedFunction() -> String {
+  if getVersion() == 1 {
+    return weakFunction()
+  }
+  return "old"
+}
+#endif

validation-test/Evolution/test_backward_deploy_always_emit_into_client.swift

@@ -0,0 +1,14 @@
+// RUN: %target-resilience-test --backward-deployment
+// REQUIRES: executable_test
+
+import StdlibUnittest
+import backward_deploy_always_emit_into_client
+
+
+var BackwardDeployTopLevelTest = TestSuite("BackwardDeployAlwaysEmitIntoClient")
+
+BackwardDeployTopLevelTest.test("BackwardDeployAlwaysEmitIntoClient") {
+  expectEqual(serializedFunction(), getVersion() == 1 ? "new" : "old")
+}
+
+runAllTests()