Idiomatic cleanup of clang/LLVM code

[jjwilke]

We are doing things that are somewhat "non-idiomatic" for LLVM. We should fix these.

• [ ] Use dyn_cast instead of checking getKind for decl/stmt types

[calewis]

Closing due to other priorities being high in the list of things to do right now. Will reopen when the time comes.