[rbi] Use interned StringRefs for diagnostics instead of SmallString<64>.

This makes the code easier to write and also prevents any lifetime issues from a
diagnostic outliving the SmallString due to diagnostic transactions.

include/swift/AST/Identifier.h

@@ -230,7 +230,7 @@ public:
 private:
   bool isOperatorSlow() const;
 };
-  
+
 class DeclName;
 class DeclNameRef;
 class ObjCSelector;