updated notes on Ubuntu llvm version ship inclusions and requirements.
[Optional Fixes #issue]
Pull Request resolved: #3769
Test Plan: Please see a detailed explanation of how to fill out the fields in the relevant sections in PULL_REQUEST.md.
Differential Revision: D18688899
Pulled By: jfix71
[Note: building Glow on Ubuntu 16.04 with llvm-7 fails because llvm-7 xenial distribution
uses an older c++ ABI]
[Note: Ubuntu 16.04 and 18.04 ship with llvm-6 and need to be upgraded before building Glow. Building Glow on Ubuntu 16.04 with llvm-7 fails because llvm-7 xenial distribution uses an older c++ ABI, however building Glow on Ubuntu 18.04 with llvm-7 has been tested and is successful]
It may be desirable to use `update-alternatives` to manage the version of