Auto merge of #3564 - alexcrichton:rustc-driver, r=phansch
Auto merge of #3564 - alexcrichton:rustc-driver, r=phansch

Link to `rustc_driver` crate in plugin

This is in anticipation for rust-lang/rust#56987 where the
`rustc_driver` crate being linked in will be required to link correctly
against the compiler. In the meantime it should be harmless otherwise!

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