When emitting complete destructors for classes with virtual bases, compute

When emitting complete destructors for classes with virtual bases, compute
the offset to the virtual bases statically inside of relying on the virtual
base offsets in the object's vtable(s).  This is both more efficient and
sound against the destructor's manipulation of the vtables.

Also extract a few helper routines.

Oh and we seem to pass all tests with an optimized clang now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96327 91177308-0d34-0410-b5e6-96231b3b80d8