Perform two more constructor/destructor code-size optimizations:

1) emit base destructors as aliases to their unique base class destructors
under some careful conditions.  This is enabled for the same targets that can
support complete-to-base aliases, i.e. not darwin.

2) Emit non-variadic complete constructors for classes with no virtual bases
as calls to the base constructor.  This is enabled on all targets and in
theory can trigger in situations that the alias optimization can't (mostly
involving virtual bases, mostly not yet supported).

These are bundled together because I didn't think it worthwhile to split them,
not because they really need to be.

llvm-svn: 96842