[docs] Add a big section with details about how to go about acquiring

a more modern host C++ toolchain for Linux distros where folks sometimes
don't have a good option to get one as part of their system.

This is a first cut, so feedback, testing, and suggestions are very,
very welcom. This is one of the last real documentation changes that was
specifically requested prior to switching LLVM and Clang to build in
C++11 mode by default.

llvm-svn: 202486