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

[Aaron Ballman 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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AaronBallman 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.

This format would make more sense to me. Basically, "Here's the principle we want to follow and why. Here's how to follow that principle." Should we also strongly encourage using `private` and `protected` access specifiers within classes too? Then the section is basically about all the ways in which you can restrict access to identifiers. (I don't know if I'm scope-creeping too much though.)

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