corpus/writer: sort emitted translation units by path name

corpus/writer: sort emitted translation units by path name

By sorting the corpora output, we achieve determinism for the emitted
XML file across multiple runs of abidw.

For that to happen, change the collection of translation units to a
std::set (instead of std::vector), sorted by absolute path name.

Test data needed adjustments, but changes are fully compatible.

	* include/abg-fwd.h: remove translation_units fwd declaration.
	* include/abg-ir.h (struct shared_translation_unit_comparator):
	Define new class.
	(translation_units): Define new typedef.
	* src/abg-corpus.cc (corpus::add): do checked insert into the
	translation_units set (rather than vector::push_back)
	* tests/data/test-annotate/test13-pr18894.so.abi: Adjust test data.
	* tests/data/test-annotate/test14-pr18893.so.abi: Likewise.
	* tests/data/test-annotate/test15-pr18892.so.abi: Likewise.
	* tests/data/test-annotate/test17-pr19027.so.abi: Likewise.
	* tests/data/test-annotate/test18-pr19037-libvtkRenderingLIC-6.1.so.abi: Likewise.
	* tests/data/test-annotate/test19-pr19023-libtcmalloc_and_profiler.so.abi: Likewise.
	* tests/data/test-annotate/test20-pr19025-libvtkParallelCore-6.1.so.abi: Likewise.
	* tests/data/test-annotate/test21-pr19092.so.abi: Likewise.

Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>