Backpatch introduction of TupleDescAttr(tupdesc, i).

Backpatch introduction of TupleDescAttr(tupdesc, i).

2cd70845240 / c6293249d 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-