Add llvm8 installation step. (#20879)
Summary:
Add ability to build docker container with llvm8.
ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20879
Differential Revision: D15497037
Pulled By: rdzhabarov
fbshipit-source-id: d673d1ddd4156c95516e61223b397c2f9bce1214
