Destructors

I refactored destructors today. The new algorithm is substantially superior in every respect- it's simpler, it's smaller, it's faster. I also introduced the CodegenContext that can make refactoring code generation easier and simpler in the future.

But the core benefit was making it EH-ready. Well, not exactly EH ready, but not too far from. I also had a quick peek at clang and CodeGenEH is only 68kb or so, which makes me feel better about the probability of Itanium EH being relatively easy to implement.

I also fixed a couple bugs and found an important and unfortunate new class of potentially compiler-crashing error. Itanium ABI says that vtable layout depends on function return type, which depends on function body, so any dependency from a member function on the vtable layout means assertion failure. I have removed dependency for calling other virtual functions but there are probably other ways in which a member function can request the contents of the vtable- constructing a new object of it's own type being the simplest example.