[analyzer] Keep track of design discusions as part of analyzer documentation.

[analyzer] Keep track of design discusions as part of analyzer documentation.

Create a directory to store discussions on potentially useful features that are
not yet implemented in the analyzer.

Fill it with a discussion on representing checker-specific parts of the program
state for C++ object modeling, that occured in D35216.

Differential Revision: https://reviews.llvm.org/D36737

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314218 91177308-0d34-0410-b5e6-96231b3b80d8