Improve effects for pairs(::NamedTuple) (#48344)
The `new` syntax as well as the implicit inner constructor
introduces an implicit `convert(fieldtype(T, n), x)` for every
field. This convert is generally a no-op (because the fieldtype
comes from the type parameter, which in turn comes from the
type of `x`). However, the compiler cannot prove this and a
`convert` call with unknown types taints all effects. Avoid
that by manually avoiding the `convert` call. It is a bit
ugly, but there isn't really another way to write it at
the moment. In the future we may want to have some nicer
way to disable the implicit generation of `convert` calls.
Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>
