Combine can't be weakly linked on Xcode 11.3

a.swift

// Run `swiftc -target x86_64-apple-macosx10.14 a.swift && nm -mg ./a`
// We will see non-weak symbol from Combine:
// `(extension in Combine):Combine.Subject< where A.Output == ()>.send() -> ()`

import Combine

@available(macOS 10.15, *)
func test() {
  let s = PassthroughSubject<Void, Never>()
  s.send()
}