[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
Centralize the handling of install_name and rpath.

This centralizes the Makefile handling of -install_name and -rpath. It also
moves the cmake build to using @rpath. The reason being that libclang needs it,
and it works for everything else.

A followup patch will move clang to using this and then there will be a single
point to edit to support other systems.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202499 91177308-0d34-0410-b5e6-96231b3b80d8