[analyzer] Teach CastSizeChecker about flexible array members.

...as well as fake flexible array members: structs that end in arrays with
length 0 or 1.

Patch by Daniel Fahlgren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201583 91177308-0d34-0410-b5e6-96231b3b80d8