I have luck here building Foundation
However, I used to set -Xlinker -FORCE:MULTIPLE to workaround linker error that says that some things was already defined.
But now I have no clues how to use it. I mean, I'm not sure what args I need to pass to swiftc to build something with Foundation 
UPDATE: I think, I figured out how to setup Foundation.
What I've done is runed cmake --build S:\b\foundation --target install and then copied all installed files from Program Files to toolchain. After that, swiftc was able to run without -I and -l{lib} flags, though I ran into another issue:
C:\SwiftDev\Dev>swiftc demo.swift -o demo.exe
e[1m<module-includes>:1:10: e[0me[0;1;30mnote: e[0me[1min file included from <module-includes>:1:
e[0m#include "CoreFoundation.h"
e[0;1;32m ^
e[0me[1mC:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\lib\swift\CoreFoundation/CoreFoundation.h:101:10: e[0me[0;1;30mnote: e[0me[1min file included from C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\lib\swift\CoreFoundation/CoreFoundation.h:101:
e[0m#include <CoreFoundation/ForSwiftFoundationOnly.h>
e[0;1;32m ^
e[0me[1mC:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\lib\swift\CoreFoundation/ForSwiftFoundationOnly.h:34:10: e[0me[0;1;31merror: e[0me[1m'fts.h' file not found
e[0m#include <fts.h>
e[0;1;32m ^
e[0me[1m<unknown>:0: e[0me[0;1;31merror: e[0me[1mcould not build C module 'CoreFoundation'
e[0m