Auto merge of #45301 - ishitatsuyuki:llvm5-backport, r=alexcrichton
Auto merge of #45301 - ishitatsuyuki:llvm5-backport, r=alexcrichton

Backport ThinLTO LLVM 5 fixes

This makes building nightly more convenient on Arch.

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