[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

@@ -1579,17 +1579,23 @@ clarification.
 
 .. _static:
 
-Anonymous Namespaces
-^^^^^^^^^^^^^^^^^^^^
-
-After talking about namespaces in general, you may be wondering about anonymous
-namespaces in particular.  Anonymous namespaces are a great language feature
-that tells the C++ compiler that the contents of the namespace are only visible
-within the current translation unit, allowing more aggressive optimization and
-eliminating the possibility of symbol name collisions.  Anonymous namespaces are
-to C++ as "static" is to C functions and global variables.  While "``static``"
-is available in C++, anonymous namespaces are more general: they can make entire
-classes private to a file.
+Restrict Visibility
+^^^^^^^^^^^^^^^^^^^
+
+Functions and variables should have the most restricted visibility possible.
+For class members, that means using appropriate `private`, `protected` or `public`
+keyword to restrict their access. For non-member functions, variables and classes,
+that means restricting visibility to a single `.cpp` file if it's not referenced
+outside that file.
+
+Visibility of file-scoped non-members variables and functions can be restricted to
+the current translation unit by using either `static` keyword or anonymous namespace.
+Anonymous namespaces are a great language feature that tells the C++ compiler that
+the contents of the namespace are only visible within the current translation unit,
+allowing more aggressive optimization and eliminating the possibility of symbol
+name collisions.  Anonymous namespaces are to C++ as `static` is to C functions and
+global variables.  While `static` is available in C++, anonymous namespaces are more
+general: they can make entire classes private to a file.
 
 The problem with anonymous namespaces is that they naturally want to encourage
 indentation of their body, and they reduce locality of reference: if you see a