Auto merge of #20894 - tigercosmos:rrr, r=avadacatavra
Auto merge of #20894 - tigercosmos:rrr, r=avadacatavra

readme add llvm clang

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update readme, since recent commit required the dependencies.
part of #20888

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jimberlage/servo,larsbergstrom/servo,saneyuki/servo,paulrouget/servo,peterjoel/servo,cbrewster/servo,nnethercote/servo,cbrewster/servo,larsbergstrom/servo,SimonSapin/servo,jimberlage/servo,nnethercote/servo,ConnorGBrewster/servo,ConnorGBrewster/servo,saneyuki/servo,KiChjang/servo,dati91/servo,szeged/servo,pyfisch/servo,paulrouget/servo,pyfisch/servo,larsbergstrom/servo,sadmansk/servo,notriddle/servo,pyfisch/servo,notriddle/servo,cbrewster/servo,emilio/servo,cbrewster/servo,peterjoel/servo,DominoTree/servo,DominoTree/servo,splav/servo,pyfisch/servo,pyfisch/servo,splav/servo,nnethercote/servo,pyfisch/servo,notriddle/servo,szeged/servo,sadmansk/servo,szeged/servo,splav/servo,splav/servo,ConnorGBrewster/servo,peterjoel/servo,nnethercote/servo,larsbergstrom/servo,sadmansk/servo,nnethercote/servo,jimberlage/servo,SimonSapin/servo,cbrewster/servo,SimonSapin/servo,dati91/servo,larsbergstrom/servo,splav/servo,szeged/servo,sadmansk/servo,larsbergstrom/servo,jimberlage/servo,nnethercote/servo,emilio/servo,saneyuki/servo,DominoTree/servo,notriddle/servo,szeged/servo,SimonSapin/servo,jimberlage/servo,sadmansk/servo,larsbergstrom/servo,jimberlage/servo,sadmansk/servo,dati91/servo,dati91/servo,ConnorGBrewster/servo,szeged/servo,peterjoel/servo,notriddle/servo,larsbergstrom/servo,KiChjang/servo,saneyuki/servo,pyfisch/servo,saneyuki/servo,KiChjang/servo,splav/servo,emilio/servo,cbrewster/servo,dati91/servo,DominoTree/servo,splav/servo,KiChjang/servo,peterjoel/servo,paulrouget/servo,saneyuki/servo,paulrouget/servo,notriddle/servo,ConnorGBrewster/servo,cbrewster/servo,KiChjang/servo,notriddle/servo,ConnorGBrewster/servo,ConnorGBrewster/servo,DominoTree/servo,paulrouget/servo,emilio/servo,pyfisch/servo,sadmansk/servo,szeged/servo,nnethercote/servo,peterjoel/servo,emilio/servo,splav/servo,paulrouget/servo,KiChjang/servo,SimonSapin/servo,DominoTree/servo,nnethercote/servo,splav/servo,dati91/servo,saneyuki/servo,notriddle/servo,SimonSapin/servo,emilio/servo,paulrouget/servo,szeged/servo,larsbergstrom/servo,KiChjang/servo,SimonSapin/servo,saneyuki/servo,jimberlage/servo,nnethercote/servo,dati91/servo,saneyuki/servo,ConnorGBrewster/servo,paulrouget/servo,szeged/servo,emilio/servo,jimberlage/servo,notriddle/servo,jimberlage/servo,szeged/servo,emilio/servo,larsbergstrom/servo,peterjoel/servo,dati91/servo,cbrewster/servo,DominoTree/servo,KiChjang/servo,peterjoel/servo,saneyuki/servo,sadmansk/servo,DominoTree/servo,emilio/servo,DominoTree/servo,pyfisch/servo,KiChjang/servo,nnethercote/servo,peterjoel/servo,splav/servo,paulrouget/servo,pyfisch/servo,notriddle/servo,emilio/servo,peterjoel/servo,DominoTree/servo,paulrouget/servo,SimonSapin/servo,KiChjang/servo