Build private swift interface files from stdlib

We have various pieces of SPI that are getting used in tests that are
failing occasionally. The swiftmodule file has the SPI, so if the
compiler loads from the swiftmodule file, everything works fine. If the
compiler loads the interface from the swiftinterface file, it fails to
find the `startOnMainActor` declaration.

Author: etcwilde

stdlib/cmake/modules/SwiftSource.cmake

@@ -640,8 +640,11 @@ function(_compile_swift_files
     if(SWIFT_ENABLE_MODULE_INTERFACES)
       set(interface_file "${module_base}.swiftinterface")
       set(interface_file_static "${module_base_static}.swiftinterface")
+      set(private_interface_file "${module_base}.private.swiftinterface")
+      set(private_interface_file_static "${module_base_static}.private.swiftinterface")
       list(APPEND swift_module_flags
-           "-emit-module-interface-path" "${interface_file}")
+           "-emit-module-interface-path" "${interface_file}"
+           "-emit-private-module-interface-path" "${private_interface_file}")
     endif()
 
     if (NOT SWIFTFILE_IS_STDLIB_CORE)
@@ -652,7 +655,7 @@ function(_compile_swift_files
     set(module_outputs "${module_file}" "${module_doc_file}")
 
     if(interface_file)
-      list(APPEND module_outputs "${interface_file}")
+      list(APPEND module_outputs "${interface_file}" "${private_interface_file}")
     endif()
 
     set(optional_arg)
@@ -720,8 +723,8 @@ function(_compile_swift_files
   set(module_outputs "${module_file}" "${module_doc_file}")
   set(module_outputs_static "${module_file_static}" "${module_doc_file_static}")
   if(interface_file)
-    list(APPEND module_outputs "${interface_file}")
-    list(APPEND module_outputs_static "${interface_file_static}")
+    list(APPEND module_outputs "${interface_file}" "${private_interface_file}")
+    list(APPEND module_outputs_static "${interface_file_static}" "${private_interface_file_static}")
   endif()
 
   swift_install_in_component(DIRECTORY "${specific_module_dir}"