bug(sql): base SQL backend discards column names

[mikegraham]

For a ColumnExpr that isn't a TableColumn, the base SQL backend unconditionally uses the name 'tmp' for the resulting query, rather than retaining the name

>>> import ibis.backends.base.sql
>>> simple_table = ibis.table(
...         [
...             ('int_col', 'int32'),
...         ],
...         'simple_table',
...     )
>>> expr = (simple_table.int_col + 3).name('foo')
>>> ibis.backends.base.sql.Compiler().to_sql(expr)
'SELECT `int_col` + 3 AS `tmp`\nFROM simple_table'

It seems like this should result in 'SELECT int_col+ 3 ASfoo\nFROM simple_table'