[docs] Add a section to the coding standards about languages and such.

A lot of this is writing down common knowledge and things often
communicated on mailing lists and in discussions. It could live in the
Programmer's Manual alternatively, but that felt slightly less
well-fitting.

It also includes (and was motivated by) the section on the relevant
language standards for LLVM and the specific features that will be
enabled with the switch to C++11.

With this, all of the documentation for the C++11 switch is, I think, in
place. I plan to flip the switch RSN. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202497 91177308-0d34-0410-b5e6-96231b3b80d8