Introduce a section to the programmers manual about type hierarchies,

Description

Introduce a section to the programmers manual about type hierarchies,
polymorphism, and virtual dispatch.

This is essentially trying to explain the emerging design techniques
being used in LLVM these days somewhere more accessible than the
comments on a particular piece of infrastructure. It covers the
"concepts-based polymorphism" that caused some confusion during initial
reviews of the new pass manager as well as the tagged-dispatch mechanism
used pervasively in LLVM and Clang.

Perhaps most notably, I've tried to provide some criteria to help
developers choose between these options when designing new pieces of
infrastructure.

Differential Revision: http://reviews.llvm.org/D7191