[libcxx-commits] [libcxx] Speed up compilation of common uses of std::visit() (PR #164196)

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higher-performance wrote:

@ldionne Thanks for the review! I applied everything except the `__upcast_to_variant` edit, since I don't think that works. It sounds like I'm fine to merge, so I'll merge this once CI is green.

https://github.com/llvm/llvm-project/pull/164196