[llvm] [CodingStandard] Add a rule about non-member definitions in CPP files (PR #126775)

[Aaron Ballman via llvm-commits]

https://github.com/AaronBallman commented:

I think we should clarify https://llvm.org/docs/CodingStandards.html#anonymous-namespaces rather than add a new section specifically. WDYT?

https://github.com/llvm/llvm-project/pull/126775