Add a target nil check

Resolves rdar://107837804

Author: elsh

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1921,7 +1921,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     (void)ID->getDecls();
 
     auto target = ID->getModule();
-    if (!getASTContext().LangOpts.PackageName.empty() &&
+    if (target && // some module might be on a block list so need to check for nil
+        !getASTContext().LangOpts.PackageName.empty() &&
         getASTContext().LangOpts.PackageName == target->getPackageName().str() &&
         !target->isNonSwiftModule() && // target is a Swift module
         target->isNonUserModule()) { // target module is in distributed SDK

test/Sema/import_package_module_from_sdk.swift

@@ -21,6 +21,10 @@
 
 // RUN: %target-swift-frontend -typecheck -verify %t/Client2.swift -package-name libPkg -I %t
 
+// RUN: rm %t/LibLocal.swiftmodule
+// RUN: not %target-swift-frontend -typecheck %t/Client2.swift -package-name libPkg -I %t  2>&1 | %FileCheck %s
+// CHECK: error: no such module 'LibLocal'
+
 //--- Lib.swift
 package func log(level: Int) {}
 
@@ -37,5 +41,3 @@ import LibLocal
 func someFunc() {
   log(level: 1)
 }
-
-