Refine IN and scalar subquery coercions

Refine IN and scalar subquery coercions

Ensure IN emits a warning when it coerces a FromClause
into a select(), however that it continues to allow the
scalar_subquery() coercion to be automatic, particularly
since it's not clear that "col IN (select)" is necessarily
"scalar" in the case of tuples.

Convert the "scalar_subquery()" warning emitted in other
cases to be a warning, rather than a deprecation warning.
I can't imagine taking this coercion out as it is intuitive
and is always going to happen; we just would like to note that
an implicit coercion is occurring.

Fixes: #5369
Change-Id: I748f01f40bc85c64e2776f9b88ef35641fa8fb5c