[llvm] [NFC] Switch a number of DenseMaps to SmallDenseMaps for speedup (PR #109417)

[Nikita Popov via llvm-commits]

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

LGTM. As long as it's not inside nested maps, there's basically no downside to having inline storage.

Also nice results on clang, so this clearly does generalize beyond CTMark:
```
bin/clang-20 	6913606M 	6858510M (-0.80%)
bin/llvm-tblgen 	261829M 	261017M (-0.31%)
bin/clang-tblgen 	121228M 	118689M (-2.09%)
```

https://github.com/llvm/llvm-project/pull/109417