Restore missing symbol _ZN7BStringixEi on x86 and x86_64.

* Vision and others were broken after 3eac8208df.
* this adds BStringRef in the backward compatibility section.
* fixes #13199.

headers/os/support/String.h

@@ -352,6 +352,7 @@ public:
 
 private:
 			class PosVect;
+			friend class BStringRef;
 
 			enum PrivateDataTag {
 				PRIVATE_DATA