[clang][dataflow] Expose getReferencedDecls and relocate free functions. (#88754)

Moves free functions from DataflowEnvironment.h/cc and
DataflowAnalysisContext.h/cc to RecordOps and a new ASTOps and exposes
them as needed for current use and to expose getReferencedDecls for
out-of-tree use.

Minimal change in functionality, only to modify the return type of
getReferenceDecls to return the collected decls instead of using output
params.

Tested with `ninja check-clang-tooling`.