[cxx-interop] Refer to SwiftBridging module from usr/include/module.modulemap

The recent Clang compiler change makes `-fno-modulemap-allow-subdirectory-search` the default behavior. This means that projects that use C++ interop and `#include <swift/bridging>` no longer compile, since Clang won't search for the SwiftBridging module under `usr/include/swift` anymore.

This change adds a new modulemap file to be installed at `toolchain/usr/include/module.modulemap`. This modulemap is under a default Clang include search path, which will make sure Clang can discover the SwiftBridging module.

rdar://123334601

Author: egorzhdan

lib/ClangImporter/CMakeLists.txt

@@ -61,9 +61,13 @@ if(SWIFT_ENABLE_CXX_INTEROP_SWIFT_BRIDGING_HEADER)
 
   swift_install_in_component(FILES
                              "${CMAKE_CURRENT_SOURCE_DIR}/bridging"
-                             "${CMAKE_CURRENT_SOURCE_DIR}/module.modulemap"
+                             "${CMAKE_CURRENT_SOURCE_DIR}/bridging.modulemap"
                              DESTINATION "include/swift"
                              COMPONENT compiler)
+  swift_install_in_component(FILES
+                             "${CMAKE_CURRENT_SOURCE_DIR}/module.modulemap"
+                             DESTINATION "include"
+                             COMPONENT compiler)
 
   add_dependencies(swiftClangImporter
       "copy_cxxInterop_support_header")

lib/ClangImporter/bridging.modulemap

@@ -0,0 +1,17 @@
+//===------------------ module.modulemap - C++ and Swift module -*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+module SwiftBridging {
+    header "bridging"
+
+    export *
+}

lib/ClangImporter/module.modulemap

@@ -10,8 +10,4 @@
 //
 //===----------------------------------------------------------------------===//
 
-module SwiftBridging {
-    header "bridging"
-
-    export *
-}
+extern module SwiftBridging "swift/bridging.modulemap"