[clang] [llvm] Remove some "DeprecatedSmallSet is deprecated" warnings from the build (PR #155407)

[Ben Langmuir via llvm-commits]

benlangmuir wrote:

Does this resolve *all* of the warnings, or just some of them? If there are still warnings it is probably better to revert the deprecation change entirely

https://github.com/llvm/llvm-project/pull/155407