Further improve the names generated for indexes on expressions.

Further improve the names generated for indexes on expressions.

Commit 181b6185c failed to do anything useful with a whole-row Var,
deeming it "fishy".  But it is legal to put such a Var into an
expression index column, so let's expand it as the name of the table.

Another problem reachable via that one is that we could generate an
empty index column name, which isn't really legal although by chance
nothing complained about it.  It's not clear whether any other such
cases remain, but as cheap insurance let's use "expr" if the tree walk
fails to generate any text.

Reported-by: Chauhan Dhruv <chauhandhruv351@gmail.com>
Author: Chauhan Dhruv <chauhandhruv351@gmail.com>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CANWwWcp_DCJjq8pomeqp6W=fbygvzXXQO028VDJ9_6sLPjQnVA@mail.gmail.com