Merge private dependencies into single static library

`Foundation` imports `CoreFoundation` with `@_implementationOnly` as a
private dependency, and it doesn't emit `IMPORTED_MODULE` in
swiftmodule.
Users of Foundation can link `libCoreFoundation.so` automatically when
using dynamic linking because it's already linked into
`libFoundation.so`.
However, this automatic linking doesn't work when static linking, so
users need to link CoreFoundation and dependent libraries of
CoreFoundation explicitly by adding `-lCoreFoundation -lBlocksRuntime
-licui18n`.

To avoid forcing users to link private dependencies explicitly, this
patch changed to merge the dependencies into libFoundation.a. And also
to avoid forcing to link dependent libs of CoreFoundation like `curl` or
`icui18n`, add `LINK_LIBRARY` entry in Foundation.swiftmodule using
`-public-autolink-library` which is introduced by
swiftlang/swift#35936

Author: kateinoigakukun

Sources/Foundation/CMakeLists.txt

@@ -160,6 +160,19 @@ set_target_properties(Foundation PROPERTIES
   Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_BINARY_DIR}/swift)
 
+if(NOT BUILD_SHARED_LIBS)
+  # TODO(katei): Comment out after swift-frontend implementation
+  # https://github.com/apple/swift/pull/35936
+  # target_compile_options(Foundation
+  #   PRIVATE
+  #     "SHELL:-public-autolink-library icui18n")
+
+  # Merge private dependencies into single static objects archive
+  set_property(TARGET Foundation PROPERTY STATIC_LIBRARY_OPTIONS
+    $<TARGET_OBJECTS:CoreFoundation>
+    $<TARGET_OBJECTS:uuid>)
+endif()
+
 if(CMAKE_SYSTEM_NAME STREQUAL Windows)
   # NOTE: workaround for CMake which doesn't link in OBJECT libraries properly
   add_dependencies(Foundation CoreFoundationResources)

Sources/FoundationNetworking/CMakeLists.txt

@@ -64,6 +64,19 @@ target_link_libraries(FoundationNetworking
     CFURLSessionInterface
   PUBLIC
     Foundation)
+
+if(NOT BUILD_SHARED_LIBS)
+  # TODO(katei): Comment out after swift-frontend implementation
+  # https://github.com/apple/swift/pull/35936
+  # target_compile_options(FoundationNetworking
+  #   PRIVATE
+  #     "SHELL:-public-autolink-library curl")
+
+  # Merge private dependencies into single static objects archive
+  set_property(TARGET FoundationNetworking PROPERTY STATIC_LIBRARY_OPTIONS
+    $<TARGET_OBJECTS:CFURLSessionInterface>)
+endif()
+
 set_target_properties(FoundationNetworking PROPERTIES
   INSTALL_RPATH "$ORIGIN"
   Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift

Sources/FoundationXML/CMakeLists.txt

@@ -16,6 +16,19 @@ target_link_libraries(FoundationXML
     CFXMLInterface
   PUBLIC
     Foundation)
+
+if(NOT BUILD_SHARED_LIBS)
+  # TODO(katei): Comment out after swift-frontend implementation
+  # https://github.com/apple/swift/pull/35936
+  # target_compile_options(FoundationXML
+  #   PRIVATE
+  #     "SHELL:-public-autolink-library xml2")
+
+  # Merge private dependencies into single static objects archive
+  set_property(TARGET FoundationXML PROPERTY STATIC_LIBRARY_OPTIONS
+    $<TARGET_OBJECTS:CFXMLInterface>)
+endif()
+
 set_target_properties(FoundationXML PROPERTIES
   INSTALL_RPATH "$ORIGIN"
   Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift