Despite being otherwise disconnected from the
constraint system, it's possible for it to affect
how we type-check tuple matches in certain cases.
This is due to the fact that:
- It can have a lower type variable ID than an
opened generic parameter type, so becomes the
representative when merged with it. And because it
has a different locator, this can influence
binding prioritization.
- Tuple subtyping is broken, as it's currently a
*weaker* relationship than conversion.
Therefore, temporarily restore this bit of logic
for language versions < 6. If possible, we should
try and fix tuple subtying in Swift 6 mode to not
accept label mismatches, so that it's not more
permissive than tuple conversion.
rdar://85263844
