[C23] Downgrade WG14 N3037 implementation status to partial (#187495)
In discussion of #186196, it started to become clear that claiming full
conformance to this feature was premature because there are still
outstanding implementation issues and standards questions with the
feature. Claiming full conformance gives users a false impression that
we think the implementation is production quality and that we don't
expect to change our implementation in breaking ways, but I believe that
we want some flexibility here and it's more accurate to say we only
partially support the feature at the moment.
Note, this is not saying we expect to break the code from that issue,
just listing where the discussion took place that led to this patch.