Add support to use llvm 7 in CI

Summary: Pull Request resolved: pytorch#12182

Differential Revision: D10129630

Pulled By: bddppq

fbshipit-source-id: f0217336474b807f03f84a4b8052ce92a6e3564b

docker/caffe2/jenkins/common/install_clang.sh

@@ -4,6 +4,13 @@ set -ex
 
 [ -n "$CLANG_VERSION" ]
 
+if [[ "$CLANG_VERSION" == "7" ]]; then
+  apt-get update
+  apt-get install -y --no-install-recommends software-properties-common wget
+  wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
+  apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main"
+fi
+
 apt-get update
 apt-get install -y --no-install-recommends clang-"$CLANG_VERSION"
 rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*