Linux symbol CRCs: support 0 and report presence changes

Linux symbol CRCs: support 0 and report presence changes

The CRC with value zero was used to mean "absent". This can be better
modelled using optional.

This commit makes this change and also tweaks reporting so that
disappearing / appearing CRCs are noted. This should be essentially
impossible unless CRCs are enabled / disabled altogether but would be
very noteworthy otherwise.

	* include/abg-ir.h (elf_symbol::elf_symbol): Argument crc is
	now an optional defaulted to absent.
	(elf_symbol::create): Likewise.
	(elf_symbol::get_crc): Now returns an optional uint64_t.
	(elf_symbol::set_src): Now takes an optional uint64_t.
	* src/abg-comp-filter.cc (crc_changed): Simplify comparison.
	* src/abg-ir.cc (elf_symbol::priv): Member crc_ is now an
	optional uint64_t.
	(elf_symbol::priv::priv): Argument crc is now an optional
	uint64_t.
	(elf_symbol::elf_symbol): Likewise.
	(elf_symbol::create): Argument crc is now an optional uint64_t
	and defaults to absent.
	(textually_equals): Simplify comparison.
	(elf_symbol::get_crc): Now returns an optional uint64_t.
	(elf_symbol::set_crc): Now takes an optional uint64_t.
	* src/abg-reader.cc (build_elf_symbol): Treat CRC 0 the same
	as other CRC values.
	* src/abg-reporter-priv.cc (maybe_report_diff_for_symbol):
	Treat CRC 0 the same as other CRC values and also report
	changes to CRC presence.
	* src/abg-writer.cc (write_elf_symbol): Treat CRC 0 the same
	as other CRC values.
	* tests/data/Makefile: Remove test-abidiff/test-crc-report.txt
	and add test-abidiff/test-crc-report-{0-1,1-0,1-2}.txt.
	* tests/data/test-abidiff/test-crc-report-0-1.txt: Report
	showing additional of CRCs.
	* tests/data/test-abidiff/test-crc-report-1-0.txt: Report
	showing removal of CRCs.
	* tests/data/test-abidiff/test-crc-report-1-2.txt: Renamed
	from tests/data/test-abidiff/test-crc-report.txt.
	* tests/test-abidiff.cc: Update test cases that no longer
	generate empty reports.
	* tests/test-symtab.cc: Update KernelSymtabsWithCRC test.

Reviewed-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Giuliano Procida <gprocida@google.com>