Rollup merge of #63170 - matklad:cleanup-fields, r=petrochenkov
Rollup merge of #63170 - matklad:cleanup-fields, r=petrochenkov

cleanup StringReader fields

reduce visibility and replace `Lrc<SourceFile>` with `start_pos`: the single bit we actually *need* from the file.

r? @petrochenkov

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