Allow changing CLR type in query translation without additional SQL

[roji]

In some rare cases, during query translation we want to change the CLR type, but without an actual SQL conversion being necessary. For example, if an enum that's value-converted to string has ToString() called on it, nothing needs to be done in SQL (the database value is already a string), but in the query tree we need to switch from the enum type to string.

Ideally we'd be able to simply replace the enum-typed node with an identical string-typed node - but there's no reliable way to change all node types (column, function call...), unless we introduce e.g. an abstract WithType() to SqlExpression.

The alternative way would be to add a special conversion node - which would extend SqlExpression - but to simply skip it in SQL generation. The disadvantage here is that any sort of pattern matching on the node would fail (e.g. is this node a column?).

In the meantime, we'd applying a SQL conversion, which should be a no-op.