Further implement defaulting constructors.

Focus is on default constructors for the time being. Currently the
exception specification and prototype are processed correctly. Codegen
might work but in all likelihood doesn't.

Note that due to an error, out-of-line defaulting of member functions is
currently impossible. It will continue to that until I muster up the
courage to admit that I secretly pray to epimetheus and that I need to
rework the way default gets from Parse -> Sema.

llvm-svn: 131115