[clang] [Serialization] Use range-based `for` loop in `GlobalModuleIndex` (NFC) (PR #214438)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

I don't know why the previous code wrotten like that. But LGTM.

https://github.com/llvm/llvm-project/pull/214438