[clang] [Rewrite] Avoid repeated hash lookups (NFC) (PR #109605)

[Nikita Popov via cfe-commits]

https://github.com/nikic approved this pull request.

LGTM, but it would probably make sense to make BlockByCopyDecls a SetVector, instead of having separate vector and set.

https://github.com/llvm/llvm-project/pull/109605