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

[Aaron Ballman via llvm-commits]

================
@@ -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`.
----------------
AaronBallman wrote:

I think this is really just clarifying the above paragraph. What if we said:
```
When you are looking at "``runHelper``" in the middle of a large C++ file,
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. For this reason, non-
member functions and variables should be marked ``static`` rather than added
to an anonymous namespace.
```

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