Rollup merge of #42266 - rkruppe:clean-trans-api, r=arielb1

Remove unused APIs from rustc_trans

There were public re-exports of some rustc modules dating back to 2011 or so. While I was at it, some functions and modules were public but never used outside the crate. I made them private or `pub(crate)` as appropriate and in one case removed an unused function.