upgrade clang_linux to LLVM 8

Released today.

Change-Id: I53132f9ff2753b8518f3d531d74a873a73455b44
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/202300
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed