[clang] [libcxx] [Clang] Add __builtin_common_reference (PR #121199)

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huixie90 wrote:

FWIW, I created patch https://github.com/llvm/llvm-project/pull/141408 to implement the libc++ counter part of the missing bit.
So can i request to merge this patch after landing of my patch? the benifits are : 
- Avoid behaviour differences between the two branches
- having more test coverage for the new behaviour that the paper introduces

https://github.com/llvm/llvm-project/pull/121199