[Concurrency] `multiple definition of 'swift::threading::fatal'` after new Threading library

[kateinoigakukun]

Describe the bug
multiple definition of 'swift::threading::fatal(char const*, ...)' linking failure happens since #59287

To Reproduce

Follow the below steps.

$ cat main.swift
import Dispatch

@main
struct Main {
  static func main() async {}
}

$ /home/katei/.ghq/github.com/kylef/swiftenv/versions/DEVELOPMENT-SNAPSHOT-2022-06-13-a/usr/bin/swiftc -parse-as-library -static-stdlib
error: link command failed with exit code 1 (use -v to see invocation)
/usr/bin/ld.gold: error: /home/katei/.ghq/github.com/kylef/swiftenv/versions/DEVELOPMENT-SNAPSHOT-2022-06-13-a/usr/lib/swift_static/linux/libswiftCore.a(Errors.cpp.o): multiple definition of 'swift::threading::fatal(char const*, ...)'
/usr/bin/ld.gold: /home/katei/.ghq/github.com/kylef/swiftenv/versions/DEVELOPMENT-SNAPSHOT-2022-06-13-a/usr/lib/swift_static/linux/libswift_Concurrency.a(ThreadingError.cpp.o): previous definition here

Expected behavior

No link failure

Environment (please complete the following information):

• OS: Ubuntu 20.04.4 LTS
• Swift Version: DEVELOPMENT-SNAPSHOT-2022-06-13-a

[kateinoigakukun]

CC: @al45tair

[al45tair]

My guess is that gold is sensitive to the link order in this case; it'll be happening because the main runtime's version of this function is implemented in the same object file as a number of other functions that are absolutely required, whereas the Concurrency runtime's version was in its own object. If the linker includes the one from the Concurrency runtime first, you'll get this error because it has to link the object from the main runtime. The fix is to pull the threading error function out into its own object in the main runtime as well, which is what I've done in #59457.

[kateinoigakukun]

OK, I've confirmed it's been fixed now. Thank you @al45tair!