docs: update minimum Clang version to 8 (#4412)
docs: update minimum Clang version to 8 (#4412)

* docs: update minimum Clang version to 8

The recent changes to enable -std=c++17 are not supported in Clang 3.8.

* Update apt instructions and symlinks too.

kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe