Rollup merge of #52472 - alexcrichton:macros-edition-2018, r=petrochenkov
Rollup merge of #52472 - alexcrichton:macros-edition-2018, r=petrochenkov

rustc: Enable `use_extern_macros` in 2018 edition

This was previously enabled via `proc_macro`, but since `proc_macro` is now
stable this is no longer the case. Explicitly include it in the 2018 edition
here.

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