Try to fix build on gcc-8 (#5430) * [machinery] Use std::forward in StructAfter Might not fix https://github.com/harfbuzz/harfbuzz/issues/5429 but I was inspecting there and looks relevant. * [machinery] Write StructAfter as an auto-return Maybe fixes https://github.com/harfbuzz/harfbuzz/issues/5429 * [machinery] Another try at fixing gcc-8 Trying to fix https://github.com/harfbuzz/harfbuzz/issues/5429