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
32bit-sparc.

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

llvm-svn: 242554