Backpatch introduction of TupleDescAttr(tupdesc, i).

2cd70845 / c6293249 change the way individual attributes in a
TupleDesc are stored / accessed.  To reduce the effort of making
extensions compatible with postgresql 11, and to ease future
backpatching, backpatch introduction of TupleDescAttr() to all
releases.  Do not backpatch change in storage, as that'd be a breaking
change for existing and working extensions.

Author: Andres Freund
Discussion: https://postgr.es/m/20170820181723.tdswdinzptbcwhrr@alap3.anarazel.de
Backpatch: 9.2-

src/include/access/tupdesc.h

@@ -81,6 +81,9 @@ typedef struct tupleDesc
 }		   *TupleDesc;
 
 
+/* Accessor for the i'th attribute of tupdesc. */
+#define TupleDescAttr(tupdesc, i) ((tupdesc)->attrs[(i)])
+
 extern TupleDesc CreateTemplateTupleDesc(int natts, bool hasoid);
 
 extern TupleDesc CreateTupleDesc(int natts, bool hasoid,