Install clang-7 from LLVM directly (#740)
Before we were installing from testing apt as clang-7 is not part of
debian stretch stable. Installing from testing introduces multiple
issues at install time of the wheels on other platforms since testing
introduces incompatible packages such as libc6=2.28 which is not
available in stable debian9/Ubuntu18.04(colab). Installing from LLVM
since there were no backports of clang-7 for deb9.
Tested mnist, cifar, and operations.
