[clang] [llvm] [NFC] Replace more DenseMaps with SmallDenseMaps (PR #111836)

[Jeremy Morse via llvm-commits]

jmorse wrote:

/me squints -- this was a minor improvement and I've lost all the context behind it, abandoning as it's not worth revisiting IMO

https://github.com/llvm/llvm-project/pull/111836