rustc_mir: promote borrows' underlying temps, and project at runtime.
rustc_mir: promote borrows' underlying temps, and project at runtime.

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