message: ci: update clang to version 7 in the Ubuntu build image.
  *Risk Level*: Low
  *Testing*: Successful build using new image.
  *Docs Changes*: n/a
  *Release Notes*: n/a
  For
diff: @@ -7,11 +7,11 @@ apt-get update
  export DEBIAN_FRONTEND=noninteractive
  apt-get install -y wget software-properties-common make cmake git python python-pip \
  bc libtool ninja-build automake zip time golang g++ gdb strace wireshark tshark
  -# clang 6.0.
  +# clang 7.
  wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
  -apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main"
  +apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main"
  apt-get update
  -apt-get install -y clang-6.0 clang-format-6.0 lld-6.0
  +apt-get install -y clang-7 clang-format-7 lld-7
  # Bazel and related dependencies.
  apt-get install -y openjdk-8-jdk curl
  echo "deb [arch=amd64] http://storage.googleapis.com/bazel-apt stable jdk1.8" | tee /etc/apt/sources.list.d/bazel.list
  