Auto merge of #43766 - michaelwoerister:trans-scheduler-touch-up, r=alexcrichton
Auto merge of #43766 - michaelwoerister:trans-scheduler-touch-up, r=alexcrichton

Improve LLVM/trans scheduling a bit

Currently it's possible that the main thread is waiting on LLVM threads to finish work while its implicit token is going to waste. This PR let's the main thread take over, so one of the running LLVM threads can free its token earlier.

r? @alexcrichton

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