[Serialization] Use the correct module for the nested type fast path. (#11018)

Fix-up for 03e1e3b, which fixes the crash caused by the new test case.

More https://bugs.swift.org/browse/SR-5284

Author: jrose-apple

lib/Serialization/Deserialization.cpp

@@ -1343,9 +1343,13 @@ ModuleFile::resolveCrossReference(ModuleDecl *baseModule, uint32_t pathLen) {
             nestedType = containingFile->lookupNestedType(memberName, baseType);
           }
         } else {
-          // Fault in extensions, then ask every serialized AST in the module.
+          // Fault in extensions, then ask every file in the module.
+          ModuleDecl *extensionModule = M;
+          if (!extensionModule)
+            extensionModule = baseType->getModuleContext();
+
           (void)baseType->getExtensions();
-          for (FileUnit *file : baseType->getModuleContext()->getFiles()) {
+          for (FileUnit *file : extensionModule->getFiles()) {
             if (file == getFile())
               continue;
             nestedType = file->lookupNestedType(memberName, baseType);

test/Serialization/Inputs/xref-nested-clang-type/NestedClangTypes.h

@@ -1,3 +1,5 @@
+#include "NestedClangTypesHelper.h"
+
 struct Outer {
   int value;
 };
@@ -6,6 +8,12 @@ struct __attribute__((swift_name("Outer.InterestingValue"))) Inner {
   int value;
 };
 
+struct OuterFromOtherModule;
+struct __attribute__((swift_name("OuterFromOtherModule.InterestingValue"))) InnerCrossModule {
+  int value;
+};
+
+
 #if __OBJC__
 
 @import Foundation;

test/Serialization/Inputs/xref-nested-clang-type/NestedClangTypesHelper.h

@@ -0,0 +1,3 @@
+struct OuterFromOtherModule {
+  int value;
+};

test/Serialization/Inputs/xref-nested-clang-type/module.modulemap

@@ -1,3 +1,9 @@
 module NestedClangTypes {
   header "NestedClangTypes.h"
+  export *
+}
+
+module NestedClangTypesHelper {
+  header "NestedClangTypesHelper.h"
+  export *
 }

test/Serialization/xref-nested-clang-type.swift

@@ -4,24 +4,25 @@
 
 // RUN: %empty-directory(%t)
 // RUN: cp %S/Inputs/xref-nested-clang-type/NestedClangTypes.h %t
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module -o %t -import-objc-header %t/NestedClangTypes.h %s -module-name Lib -DNO_MODULE
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %t -import-objc-header %t/NestedClangTypes.h %s -DCLIENT -verify
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module -o %t -import-objc-header %t/NestedClangTypes.h -I %S/Inputs/xref-nested-clang-type/ %s -module-name Lib -DNO_MODULE
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %t -I %S/Inputs/xref-nested-clang-type/ -import-objc-header %t/NestedClangTypes.h %s -DCLIENT -verify
 
 // RUN: %empty-directory(%t)
 // RUN: cp %S/Inputs/xref-nested-clang-type/NestedClangTypes.h %t
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module -o %t -import-objc-header %t/NestedClangTypes.h -pch-output-dir %t/PCHCache %s -module-name Lib -DNO_MODULE
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %t -pch-output-dir %t/PCHCache -import-objc-header %t/NestedClangTypes.h %s -DCLIENT -verify
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module -o %t -import-objc-header %t/NestedClangTypes.h -I %S/Inputs/xref-nested-clang-type/ -pch-output-dir %t/PCHCache %s -module-name Lib -DNO_MODULE
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %t -I %S/Inputs/xref-nested-clang-type/ -pch-output-dir %t/PCHCache -import-objc-header %t/NestedClangTypes.h %s -DCLIENT -verify
+
+#if _runtime(_ObjC)
+import Foundation
+#endif // ObjC
 
 #if CLIENT
 
 import Lib
 
 func test(x: MyInner) {}
-
-#if _runtime(_ObjC)
-import Foundation
+func test(x: MyOtherInner) {}
 func test(x: MyCode) {}
-#endif // ObjC
 
 #else // CLIENT
 
@@ -36,13 +37,18 @@ extension MyOuter {
   public func use(inner: MyInner) {}
 }
 
-#if _runtime(_ObjC)
-import Foundation
+public typealias MyOtherInner = OuterFromOtherModule.InterestingValue
+extension OuterFromOtherModule {
+  public func use(inner: MyOtherInner) {}
+}
 
+#if _runtime(_ObjC)
 public typealias MyCode = ErrorCodeEnum.Code
 extension ErrorCodeEnum {
   public func whatever(code: MyCode) {}
 }
+#else
+public typealias MyCode = Int
 #endif // ObjC
 
 #endif // CLIENT