Enable ubsan in darwin unit tests.

[bzbarsky-apple]

1. Fix some ids that should have been marked _Nullable to say that.
2. Change the attribute/event decoder codegen to not generate a single huge function: with both asan and ubsan enabled, this caused the function to use too much stack for some of the instrumentation and run out of stack space in the thread.
3. Remove some commented-out debugging code that crept into MTRDeviceTests.
4. Enable UndefinedBehaviorSanitizer in the Xcode test scheme. 5) Lay the groundwork for also building libCHIP with ubsan when doing these tests. Not enabled yet because it runs into some issues so far.

[github-actions]

PR #26467: Size comparison from 65a3b38 to 9edc4d3

Decreases (1 build for cc32xx)

platform	target	config	section	65a3b38	9edc4d3	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	19674627	19674626	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	65a3b38	9edc4d3	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	605002	605002	0	0.0
			(read/write)	204164	204164	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197576	197576	0	0.0
			.comment	206	206	0	0.0
			.data	1468	1468	0	0.0
			.debug_abbrev	957619	957619	0	0.0
			.debug_aranges	101232	101232	0	0.0
			.debug_frame	342080	342080	0	0.0
			.debug_info	19674627	19674626	-1	-0.0
			.debug_line	2671903	2671903	0	0.0
			.debug_line_str	513	513	0	0.0
			.debug_loc	33340	33340	0	0.0
			.debug_loclists	1491139	1491139	0	0.0
			.debug_ranges	4984	4984	0	0.0
			.debug_rnglists	94412	94412	0	0.0
			.debug_str	3131828	3131828	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104314	104314	0	0.0
			.shstrtab	265	265	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	483089	483089	0	0.0
			.symtab	287232	287232	0	0.0
			.text	498564	498564	0	0.0