[analyzer] Add an internal reference document describing IPA and CallEvent.

This attempts to be a higher-level description of our inlining heuristics
and decision trees than the source, where the work is spread out between
ExprEngine (mostly in ExprEngineCallAndReturn.cpp) and CallEvent, with a
few other classes participating as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162073 91177308-0d34-0410-b5e6-96231b3b80d8