Rollup merge of #48124 - alexcrichton:clean-up-debugging, r=kennytm
Rollup merge of #48124 - alexcrichton:clean-up-debugging, r=kennytm

Explain unusual debugging code in librustc

Introduced in #47828 to help track down some bugs, it landed a bit hastily so
this is intended on cleaning it up a bit.

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