Implement Weak::new_downgraded() (#30425)
Implement Weak::new_downgraded() (#30425)

This adds a constructor for a Weak that can never be upgraded. These are
mostly useless, but for example are required when deserializing.

graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust