[llvm-objcopy] IHexELFBuilder::addDataSections - fix evaluation ordering static analyzer warning

As detailed on https://pvs-studio.com/en/blog/posts/cpp/0771/ and raised on D62583, the SecNo++ increment is not guaranteed to occur before the second use of SecNo in the same addSection() call.

This patch pulls out the increment (just for clarity) and replaces the second use of SecNo with a constant zero value (we're using stable_sort so the value isn't critical).

Differential Revision: https://reviews.llvm.org/D107273