BUG #15775: pg_get_indexdef: could not open relation with OID 16385

The following bug has been logged on the website:

Bug reference:      15775
Logged by:          Pengzhou Tang
Email address:      ptang@pivotal.io
PostgreSQL version: 11.2
Operating system:   centos 7
Description:

Hi, Developers,

I hit this error when running bellow queries concurrently:
1. select pg_get_indexdef('idx_foo'::regclass);
2. drop table foo;

To repo easier, break at set_relation_column_names in session 1 and then run
session 2, error occurs.

Question:
Why pg_get_expr() doesn't acquire and hold an AccessShareLock if it needs to
access the relation later.