feat(compiler): Support subqueries in the FROM clause

[Jille]

issue #2989, #2400 and probably others

This is without #3220 so we can separate which test failures are caused by which PR.

[Jille]

I've added a fix for unnamed subqueries. That panicked at HEAD and my branch, and works now :)

[alexrjones]

@Jille, I think the reason why 7d1b0c6 was required is because this test uses an active database to perform type inference for the query. All tests that use the managed-db config mutate the config to connect to the DB:

sqlc/internal/endtoend/endtoend_test.go

Lines 121 to 144 in e8405b5

	Mutate: func(t *testing.T, path string) func(*config.Config) {
	return func(c *config.Config) {
	for i := range c.SQL {
	files := []string{}
	for _, s := range c.SQL[i].Schema {
	files = append(files, filepath.Join(path, s))
	}
	switch c.SQL[i].Engine {
	case config.EnginePostgreSQL:
	uri := local.ReadOnlyPostgreSQL(t, files)
	c.SQL[i].Database = &config.Database{
	URI: uri,
	}
	case config.EngineMySQL:
	uri := local.MySQL(t, files)
	c.SQL[i].Database = &config.Database{
	URI: uri,
	}
	default:
	// pass
	}
	}
	}
	},

Here is where analysis is performed for columns that are not part of a table:

sqlc/internal/engine/postgresql/analyzer/analyze.go

Lines 272 to 288 in caaaacc

	dataType, err := a.formatType(ctx, field.DataTypeOID, field.TypeModifier)
	if err != nil {
	return nil, err
	}
	// debug.Dump(i, field, dataType)
	notNull := false
	name := field.Name
	dt, isArray, dims := parseType(dataType)
	result.Columns = append(result.Columns, &core.Column{
	Name: name,
	OriginalName: field.Name,
	DataType: dt,
	NotNull: notNull,
	IsArray: isArray,
	ArrayDims: int32(dims),
	})
	}

Notice that notNull is always false in this block. So, for people who do use this setup, the CI output would match this output. I believe this is the reason for the difference between your local sqlc generate output and the CI output, but I'm not sure what can be changed here to allow this analyzer to infer the nullability of this column.