[Serialization] Fix deserialization crash occurring when a mixed framework fails to load its ObjC part.

lib/Serialization/ModuleFile.cpp

@@ -1093,7 +1093,7 @@ Status ModuleFile::associateWithFileContext(FileUnit *file,
              "invalid module name (submodules not yet supported)");
     }
     auto module = getModule(modulePath);
-    if (!module) {
+    if (!module || module->failedToLoad()) {
       // If we're missing the module we're shadowing, treat that specially.
       if (modulePath.size() == 1 &&
           modulePath.front() == file->getParentModule()->getName()) {

test/Serialization/Inputs/MixModA.modulemap

@@ -0,0 +1,8 @@
+framework module MixModA {
+  export *
+  module * { export * }
+}
+
+module MixModA.Swift {
+    header "MixModA-Swift.h"
+}

test/Serialization/Inputs/SwiftModA.swift

@@ -0,0 +1,5 @@
+import ObjCFail
+
+open class SwiftClsA {
+  public init() {}
+}

test/Serialization/Inputs/SwiftModB.swift

@@ -0,0 +1,5 @@
+import MixModA
+
+open class TyB : SwiftClsA {
+  public override init() { super.init() }
+}

test/Serialization/Inputs/objcfail/module.modulemap

@@ -0,0 +1,3 @@
+module ObjCFail {
+  header "objcfail.h"
+}

test/Serialization/Inputs/objcfail/objcfail.h

@@ -0,0 +1,3 @@
+#ifdef FAIL
+#error failing clang module
+#endif

test/Serialization/failed-clang-module.swift

@@ -0,0 +1,19 @@
+// Test that there is no crash in such a case:
+// - there is mixed framework A
+// - swift module B depends on A and is built fine
+// - there is a swift invocation that imports B but causes the ObjC part of A to fail to import
+
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t/MixModA.framework/Headers
+// RUN: mkdir -p %t/MixModA.framework/Modules/MixModA.swiftmodule
+// RUN: cp %S/Inputs/MixModA.modulemap %t/MixModA.framework/Modules/module.modulemap
+
+// RUN: %target-swift-frontend -emit-module %S/Inputs/SwiftModA.swift -module-name MixModA -I %S/Inputs/objcfail -o %t/MixModA.framework/Modules/MixModA.swiftmodule/x86_64.swiftmodule -emit-objc-header -emit-objc-header-path %t/MixModA.framework/Headers/MixModA-Swift.h -module-cache-path %t/mcp
+// RUN: %target-swift-frontend -emit-module %S/Inputs/SwiftModB.swift -module-name SwiftModB -F %t -o %t -module-cache-path %t/mcp
+
+// RUN: %target-swift-frontend -parse %s -I %t -module-cache-path %t/mcp
+// RUN: %target-swift-frontend -parse %s -Xcc -DFAIL -I %t -module-cache-path %t/mcp -show-diagnostics-after-fatal -verify
+
+import SwiftModB // expected-error {{missing required module}}
+_ = TyB() // expected-error {{use of unresolved identifier 'TyB'}}