[clang] [CIR][CodeGen][NFC] Move ast helper function to FunctionUtils (PR #226877)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

FunctionUtils is for things that both classic and CIR codegen share.   I don't see this being used in the classic-codegen, so I don't really think this is time for this yet.

https://github.com/llvm/llvm-project/pull/226877