Auto merge of #124728 - beetrees:from-f16-for-f64, r=BurntSushi

Re-add `From<f16> for f64`

This impl was originally added in #122470 before being removed in #123830 due to #123831. However, the issue only affects `f32` (which currently only has one `From<{float}>` impl, `From<f32>`) as `f64` already has two `From<{float}>` impls (`From<f32>` and `From<f64>`) and is also the float literal fallback type anyway. Therefore it is safe to re-add `From<f16> for f64`.

This PR also updates the FIXME link to point to the open issue #123831 rather than the closed issue #123824.

Tracking issue: #116909

`@rustbot` label +F-f16_and_f128 +T-libs-api

Author: bors