Add documentation and tests for Clangs C++11 extensions in C++03.

As we gear up to drop support for GCC in C++03, we should make clear
what our C++03 mode is, the C++11 extensions it provides,
and the C++11 extensions it depends on.

The section of this document discussing user-facing extensions has
been left blank while the community discusses new directions. For now
it's just a warning to users.

Additionally, the document contains examples of how these extensions
should be used and why. For example, using alias templates over class
templates.

llvm-svn: 363110