[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

@@ -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`.
+
+
 Don't Use Braces on Simple Single-Statement Bodies of if/else/loop Statements
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^