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

[Rahul Joshi via llvm-commits]

jurahul wrote:

> Sound good. This note is about *what* to internalize and the anonymous namespace is about *how* but I can move this line to the end of that section.
Moved it to the end of the anonymous namespaces section. PTAL.

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