Add support for diagnostic tags, and use them for `mergeDefaultSetupAndUserConfigs` #10499

Job	Run time
Split workflow (macos-latest, linked)	1m 33s
Split workflow (ubuntu-latest, nightly-latest)	1m 24s
Split workflow (ubuntu-latest, linked)	1m 16s
Split workflow (macos-latest, default)	1m 36s
Split workflow (macos-latest, nightly-latest)	1m 37s
Split workflow (ubuntu-latest, default)	1m 11s
	8m 37s

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