Rollup merge of #145007 - Kobzol:error-index, r=jieyouxu

Fix build/doc/test of error index generator

It is essentially a RustcPrivate tool, so it should be treated as such using the new `RustcPrivateCompilers` infra. Found while working on unrelated `doc` cleanups.

r? ````@jieyouxu````

Author: tgross35