opt: attempting to aggregate by an outer column causes an internal error

It is possible to trigger assertion errors like the following during optimization by attempting to aggregate over an outer-column reference:
```
expected success, but found
(XX000) internal error: top-level relational expression cannot have outer columns: (4)
optimizer.go:276: in Optimize()
DETAIL: stack trace:
pkg/sql/opt/xform/optimizer.go:276: Optimize()
pkg/sql/plan_opt.go:862: buildExecMemo()
pkg/sql/plan_opt.go:260: makeOptimizerPlan()
pkg/sql/conn_executor_exec.go:3302: makeExecPlan()
pkg/sql/conn_executor_exec.go:2848: dispatchToExecutionEngine()
pkg/sql/conn_executor_exec.go:2115: execStmtInOpenStateWithPausablePortal()
pkg/sql/conn_executor_exec.go:163: func1()
pkg/sql/conn_executor_exec.go:4463: execWithProfiling()
pkg/sql/conn_executor_exec.go:162: execStmt()
pkg/sql/conn_executor.go:2425: func1()
pkg/sql/conn_executor.go:2430: execCmd()
pkg/sql/conn_executor.go:2347: run()
pkg/sql/conn_executor.go:1018: ServeConn()
pkg/sql/pgwire/conn.go:252: processCommands()
pkg/sql/pgwire/server.go:1197: func4()
src/runtime/asm_arm64.s:1223: goexit()
```
Here are two logic tests that reproduce the issue:

One with the outer column produced by an apply-join:
```
statement ok
SELECT * FROM (SELECT 1) AS foo(bar)
INNER JOIN LATERAL (SELECT sum(bar) FROM generate_series(1, 10)) ON true
```
And one with the outer column produced by a routine:
```
statement ok
CREATE FUNCTION bar() RETURNS DECIMAL LANGUAGE PLpgSQL AS $$
DECLARE
baz DECIMAL := 1.0;
BEGIN
SELECT sum(baz) INTO baz
FROM generate_series(1, 10) g(t);
RETURN baz;
END
$$;

statement ok
SELECT bar()
```
Here's what Postgres does for the first example:
```
postgres=# SELECT * FROM (SELECT 1) AS foo(bar)
INNER JOIN LATERAL (SELECT sum(bar) FROM generate_series(1, 10)) ON true;
ERROR: 42803: aggregate functions are not allowed in FROM clause of their own query level
LINE 2: INNER JOIN LATERAL (SELECT sum(bar) FROM generate_series(1, ...
^
LOCATION: check_agglevels_and_constraints, parse_agg.c:589
```
Postgres actually allows the routine example, but it's unclear if we want to do the same:
```
postgres=# CREATE FUNCTION bar() RETURNS DECIMAL LANGUAGE PLpgSQL AS $$
DECLARE
baz DECIMAL := 1.0;
BEGIN
SELECT sum(baz) INTO baz
FROM generate_series(1, 10) g(t);
RETURN baz;
END
$$;
CREATE FUNCTION
postgres=# SELECT bar();
bar
------
10.0
(1 row)
```

Jira issue: CRDB-48542