[llvm-rc] Exclude padding from sizes in versioninfo resources

Normally when writing something that requires padding, we first
measure the length of the written payload data, then write
padding if necessary.

For a recursive structure like versioninfo, this means that the
padding is excluded from the size of the inner element, but
included in the size of the enclosing block.

Rc.exe excludes the final padding (but not the padding of earlier
children) from all levels of the hierarchy.

To achieve this, don't pad after each block or value, but only
before starting the next one. We still pad after completing the
toplevel versioninfo resource, so this won't affect other resource
types.

Differential Revision: https://reviews.llvm.org/D46510

llvm-svn: 331668