The _Builtin_float symbols were moved twice, most recently from the OS Darwin library, but previously they were in the embedded @rpath Darwin library. @_originallyDefinedIn doesn't support multiple install names, but it can be replaced with -module-abi-name, and then multiple $ld$previous$ symbols can be used.
Update the Platform and Concurrency magic symbols to use $ld$previous$ everywhere.
rdar://130107191
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
With these magic symbols, programs that link against the _Concurrency
module with a deployment target prior to iOS 15 / macOS 12 / watchOS 8
will reference `libswift_Concurrency.dylib` via rpath.
Fixes rdar://81187835.
