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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

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

I think the code is simply quite old, from commit e060e57bf74681939ffab90ff743ab8594a8b866 in 2013 when LLVM probably didn't have C++11 range-based `for` loops yet...

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