Switch xla job to use bionic clang9 image (#36618)
Summary:
XLA need to switch to clang9 to build with latest TF dependency.
We keep pytorch/pytorch build remain using gcc for now.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36618
Differential Revision: D21045723
Pulled By: ailzhang
fbshipit-source-id: 015b65dad2aeef31fd66b753d519b2c9b9ed8b7f
