[clang] [Clang] remove redundant uses of dyn_cast (NFC) (PR #189106)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Nice! I Expected a lot more, hopefully we won't see much back sliding. It would have been interesting to see a distribution of when they were added (in time).

A more interesting analysis but harder (I think) would be how much review each one got. I would love for people to do this kind of archelogy b/c we would likely see a lot of telling patterns.

https://github.com/llvm/llvm-project/pull/189106