[llvm] [Bitcode] Use DenseSet instead of std::set (NFC) (PR #105851)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> lgtm but can you fix the description - it is switching from std::set not std::map

Thanks!  Fixed.

https://github.com/llvm/llvm-project/pull/105851