[LLVM][CodingStandard] Extend namespace qualifier rule to variables/classes (llvm#163588)

Extend CS rule to use namespace qualifiers to define previously declared
functions to variables and classes as well.

Author: jurahul

llvm/docs/CodingStandards.rst

@@ -860,27 +860,40 @@ your private interface remains private and undisturbed by outsiders.
     It's okay to put extra implementation methods in a public class itself. Just
     make them private (or protected) and all is well.
 
-Use Namespace Qualifiers to Implement Previously Declared Functions
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Use Namespace Qualifiers to Define Previously Declared Symbols
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-When providing an out-of-line implementation of a function in a source file, do
-not open namespace blocks in the source file. Instead, use namespace qualifiers
-to help ensure that your definition matches an existing declaration. Do this:
+When providing an out-of-line definition for various symbols (variables,
+functions, opaque classes) in a source file, do not open namespace blocks in the
+source file. Instead, use namespace qualifiers to help ensure that your
+definition matches an existing declaration. Do this:
 
 .. code-block:: c++
 
   // Foo.h
   namespace llvm {
+  extern int FooVal;
   int foo(const char *s);
-  }
+
+  namespace detail {
+  class FooImpl;
+  } // namespace detail
+  } // namespace llvm
 
   // Foo.cpp
   #include "Foo.h"
   using namespace llvm;
+
+  int llvm::FooVal;
+
   int llvm::foo(const char *s) {
     // ...
   }
 
+  class detail::FooImpl {
+    // ...
+  }
+
 Doing this helps to avoid bugs where the definition does not match the
 declaration from the header. For example, the following C++ code defines a new
 overload of ``llvm::foo`` instead of providing a definition for the existing