BUG #16702: inline code and function : when use dynamic name for rowtype, there is some bug!

The following bug has been logged on the website:

Bug reference: 16702
Logged by: Zhou Digoal
Email address: digoal(at)126(dot)com
PostgreSQL version: 12.4
Operating system: CentOS 7.7 x64
Description:

postgresql 12, when i use dynamic name for rowtype, there is some bug!

```
postgres=> \d b
Table "public.b"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
id | integer | | |
c1 | jsonb | | |
Indexes:
"idx_b_1" gin (c1)

postgres=> \d a
Table "public.a"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
id | bigint | | not null |
gid | integer | | |
score | integer | | |
info | text | | |
Indexes:
"a_pkey" PRIMARY KEY, btree (id)
"idx_a_1" btree (gid, score)
"idx_a_2" btree (gid)

postgres=> \d tbl
Table "public.tbl"
Column | Type | Collation | Nullable | Default
----------+-----------------------------+-----------+----------+---------
col | integer | | |
c1 | integer | | |
c2 | integer | | |
mod_time | timestamp without time zone | | |

postgres=> select * from a limit 1;
id | gid | score | info
----+--------+-------+----------------------------------
1 | 112736 | 393 | 3d41b33b5e739b30eebfa15109e2db9f
(1 row)

postgres=> select * from tbl limit 1;
col | c1 | c2 | mod_time
------+----+----+----------------------------
9150 | 32 | 47 | 2020-10-31 17:06:28.452212
(1 row)

postgres=> do language plpgsql $$
declare
y text := 'tbl';
b y%rowtype;
begin
select tbl.* into b from tbl limit 1;
raise notice '%', b;
end;
$$;
ERROR: relation "y" does not exist
CONTEXT: compilation of PL/pgSQL function "inline_code_block" near line 4

postgres=> do language plpgsql $$
declare
a text := 'tbl';
b a%rowtype;
begin
select tbl.* into b from tbl limit 1;
raise notice '%', b;
end;
$$;
NOTICE: (9150,32,47,"2020-10-31 17:06:28.452212")
DO

postgres=> create or replace function f(a text) returns void as $$

declare v a%rowtype;
begin
execute format('select * from %I limit 1', a) into v;
raise notice '%: %', a, v;
end;
$$ language plpgsql strict;
CREATE FUNCTION

postgres=> select * from f('tbl');
NOTICE: tbl: (9150,32,47,"2020-10-31 17:06:28.452212")
f
---

(1 row)

postgres=> select * from f('b');
ERROR: invalid input syntax for type integer: "[{"a": 1, "b": 2}, {"c": 2,
"d": 4}]"
CONTEXT: PL/pgSQL function f(text) line 4 at EXECUTE
```

i know there only check the name(variable name) is exists , but when execute
, it use the dynamic name(variable value) for it , not the static
name(variable name).