[clang] [randstruct] Also randomize composite function pointer structs (PR #138385)

[Bill Wendling via cfe-commits]

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

Looks fine. Though I can't shake the feeling that randomization should happen in the `ActOnTagFinishDefinition` method.

https://github.com/llvm/llvm-project/pull/138385