[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202486 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Tweak discussion of BSDs based on feedback from Roman Divacky.
FreeBSD 10.0 and newer have a modern Clang toolchain that should work
well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202488 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Fix 80-column wrap that I messed up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202489 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Fix my links to use the correct ReST syntax.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202490 91177308-0d34-0410-b5e6-96231b3b80d8