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[6.0][Runtime] Avoid dlsym of objc_isUniquelyReferenced.
Cherry-pick https://github.com/apple/swift/pull/73285 to release/6.0.
We attempted to use the declaration if it exists, and fall back to dlsym. This didn't actually work and we always call dlsym. Include the right header (when available) and add a weak check to the direct call.
rdar://127116080
Explanation: We want to directly reference objc_isUniquelyReferenced when it's available in the SDK, and look it up with dlsym when it isn't. The previous attempt to do that didn't quite work, and always used dlsym, which imposes a performance penalty on first use..
Original PR: https://github.com/apple/swift/pull/73285
Reviewed by: @Catfish-Man
Risk: Low. We make the same call, but in the normal way by referencing it from a header, instead of looking it up with dlsym.
Resolves: rdar://127116080
Tests: Many existing tests exercise this code.