Remove unused prototype.
Respect type alignment when choosing padding fields.

Fixes ABI mismatch when e.g. padding a 28 byte union from a
20 byte member, where previously, an i64 would be emitted,
yielding a 32 byte LLVM struct size on ulong.alignof == 8
platforms.

Test case will follow on the 2.064 branch.
Merge pull request #556 from klickverbot/padding-fix

Respect type alignment when choosing zero fill types.