[Serialization] Don't serialize an internally-imported bridging header

Internally-imported bridging headers must not leak outside of the Swift
module. Don't serialize their contents, and make sure we can still
import the module even if the bridging header has been removed.

Author: DougGregor

lib/Frontend/Frontend.cpp

@@ -197,7 +197,8 @@ SerializationOptions CompilerInvocation::computeSerializationOptions(
   serializationOpts.DocOutputPath = outs.ModuleDocOutputPath;
   serializationOpts.SourceInfoOutputPath = outs.ModuleSourceInfoOutputPath;
   serializationOpts.GroupInfoPath = opts.GroupInfoPath.c_str();
-  if (opts.ModuleHasBridgingHeader && !outs.ModuleOutputPath.empty())
+  if (opts.ModuleHasBridgingHeader && !outs.ModuleOutputPath.empty() &&
+      !opts.ImportHeaderAsInternal)
     serializationOpts.SerializeBridgingHeader = true;
   // For batch mode, emit empty header path as placeholder.
   if (serializationOpts.SerializeBridgingHeader &&

test/ClangImporter/InternalBridgingHeader/serialization.swift

@@ -0,0 +1,48 @@
+// RUN: %empty-directory(%t)
+
+// Stage in the headers we need
+// RUN: mkdir %t/headers
+// RUN: cp %S/../Inputs/c-bridging-header.h %t/headers
+
+// RUN: mkdir %t/src
+// RUN: split-file %s %t/src
+
+// Build a module
+// RUN: mkdir %t/modules
+// RUN: %target-swift-frontend -internal-import-bridging-header %t/headers/c-bridging-header.h -sdk %clang-importer-sdk -emit-module -o %t/modules/MyModule.swiftmodule %t/src/MyModule.swift
+
+// Check that there's no serialized bridging header in the module file.
+// RUN: llvm-bcanalyzer -dump %t/modules/MyModule.swiftmodule | %FileCheck -check-prefix MODULE-FILE %s
+
+// Use the module.
+// RUN: %target-swift-frontend -typecheck -sdk %clang-importer-sdk -I %t/modules %t/src/MyClient.swift
+
+// Delete the bridging header, and again use the module.
+// RUN: rm %t/headers/c-bridging-header.h
+// RUN: %target-swift-frontend -typecheck -sdk %clang-importer-sdk -I %t/modules %t/src/MyClient.swift
+
+//--- MyModule.swift
+func getRed() -> Color { red }
+
+func getX(point: MyPoint) -> Double { point.x }
+
+public func f() {
+  _ = getRed()
+}
+
+//--- MyClient.swift
+
+import MyModule
+
+func g() {
+  f()
+}
+
+
+// MODULE-FILE-NOT: IMPORTED_HEADER
+// MODULE-FILE-NOT: IMPORTED_HEADER_CONTENTS
+
+// MODULE-FILE: SEARCH_PATH
+
+// MODULE-FILE-NOT: IMPORTED_HEADER
+// MODULE-FILE-NOT: IMPORTED_HEADER_CONTENTS