[clang] [clang][Analysis] Speed up LiveVariables set merge (PR #209430)

[Balázs Benics via cfe-commits]

steakhal wrote:

While this made it into release/23.x, this is the only entry missing from the CSA release notes. I don't think it's worth the effort referencing this PR from the notes because it doesn't seem to do much for the static analyzer anyway.
I think it's fine.

https://github.com/llvm/llvm-project/pull/209430