Any update on pre-build toolchains with Foundation? 
I have successfully build it, but I was not able to use it. It seems I have to specify some flags to swiftc so that the Clang Importer can correctly read CoreFoundation headers.
Z:\Downloads>swiftc main.swift -o main.exe && main
<module-includes>:1:10: note: in file included from <module-includes>:1:
e#include "CoreFoundation.h"
^
C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\lib\swift\CoreFoundation/CoreFoundation.h:101:10: note: in file included from C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\lib\swift\CoreFoundation/CoreFoundation.h:101:
#include <CoreFoundation/ForSwiftFoundationOnly.h>
^
C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\lib\swift\CoreFoundation/ForSwiftFoundationOnly.h:33:10: error: 'fts.h' file not found
#include <fts.h>
^
<unknown>:0: error: could not build C module 'CoreFoundation'