[CodingStandard] Add a rule about non-member definitions in CPP files

- Add a rule suggesting that non-member definitions in CPP files should
  be made static unless they are referenced outside that file.

Author: jurahul

llvm/docs/CodingStandards.rst

@@ -1499,6 +1499,12 @@ Do:
     }
   };
 
+Make non-member variables and functions in `.cpp` files static
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When non-member functions or variables are defined in a `.cpp`` file and need
+not be referenced outside that file, make them `static`.
+
 Microscopic Details
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