namedtuple: Use correct return type in merge/diff (#48290)
In a lapse of memory, I had assumed that NamedTuple was covariant
like Tuple, but since this is not the case, we do actually need
to pass the types into the constructor. However, the main
constructor for NamedTuple has an extra `convert` call to the
declared tuple type. This call is problematic for effects, because
the type is unknown. For the merge/diff case, we are
guaranteed that the convert is a no-op, but the compiler's analysis
is not strong enough to prove this. Work around that by
introducing an `_NamedTuple` constructor that bypasses the
unnecessary convert to make sure that the compiler can prove
sufficiently strong effects.
