These tests were targeting 10.9 or 10.10, but the minimum deployment target now supported by Xcode is 10.13. Bump them up to match:
test/Concurrency/Backdeploy/linking.swift
test/Concurrency/Backdeploy/linking_maccatalyst.swift
test/Runtime/stable-bit-backward-deployment.swift
Most of the changes fall into a few categories:
* Replace explicit "x86_64" with %target-cpu in lit tests
* Cope with architecture differences in IR/asm/etc. macOS-specific tests
Recent Swift uses 2 as the is-Swift bit when running on newer versions, and 1 on older versions. Since it's difficult or impossible to know what we'll be running on at build time, make the selection at runtime.