Rollup merge of #50148 - japaric:const-manuallydrop, r=oli-obk
Rollup merge of #50148 - japaric:const-manuallydrop, r=oli-obk

turn `ManuallyDrop::new` into a constant function

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