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

[Rahul Joshi via llvm-commits]

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@@ -1640,6 +1640,10 @@ you have no immediate way to tell if this function is local to the file.  In
 contrast, when the function is marked static, you don't need to cross-reference
 faraway places in the file to tell that the function is local.
 
+Related to this issue, when non-member functions or variables are defined in a
+`.cpp`` file and need not be referenced outside that file, make them `static`.
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jurahul wrote:

Or to belabor on this more, may be this should be the first paragraph, saying that things should be as private as possible, and then going into how to achieve that via static or anon namespaces. The section can be renamed to something like *privatize aggressively* or keep it as is.

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