Fix alignment issues in Clang.
Some const-correctness changes snuck in here too, since they were in the
area of code I was modifying.
This seems to make Clang actually work without Bus Error on
Follow-up patches will factor out a trailing-object helper class, to
make classes using the idiom of appending objects to other objects
easier to understand, and to ensure (with static_assert) that required
alignment guarantees continue to hold.
Differential Revision: http://reviews.llvm.org/D10272