update documentation for Ubuntu build (#2825)

Summary:
*Description*: updating Ubuntu build documentation with llvm-8 as tested on CI
*Testing*: CI
Pull Request resolved: #2825

Differential Revision: D15350270

Pulled By: bertmaher

fbshipit-source-id: 06027467e8243ed11d61de5c6796c8aab1ac8567

README.md

@@ -122,19 +122,21 @@ In order to build Glow on Ubuntu it is necessary to install a few packages. The
 following command should install the required dependencies:
 
   ```bash
-  sudo apt-get install clang clang-6.0 cmake graphviz libpng-dev \
-      libprotobuf-dev llvm-6.0 ninja-build protobuf-compiler wget \
+  sudo apt-get install clang clang-8 cmake graphviz libpng-dev \
+      libprotobuf-dev llvm-8 llvm-8-dev ninja-build protobuf-compiler wget \
       opencl-headers libgoogle-glog-dev
   ```
+[Note: building Glow on Ubuntu 16.04 with llvm-7 fails because llvm-7 xenial distribution
+uses an older c++ ABI]
 
 It may be desirable to use `update-alternatives` to manage the version of
 clang/clang++:
 
   ```bash
   sudo update-alternatives --install /usr/bin/clang clang \
-      /usr/lib/llvm-6.0/bin/clang 50
+      /usr/lib/llvm-8/bin/clang 50
   sudo update-alternatives --install /usr/bin/clang++ clang++ \
-      /usr/lib/llvm-6.0/bin/clang++ 50
+      /usr/lib/llvm-8/bin/clang++ 50
   ```
 
 Glow uses the system default C/C++ compiler (/usr/bin/c++), and so you may also