Go for clang-15 to work around GH's Clang SNAFU #1510

Job	Run time
CI (ubuntu-22.04, 6.4, e2ee)	3m 47s
CI (ubuntu-22.04, 6.4)	1m 49s
CI (ubuntu-22.04, 6.4, GCC, e2ee)	10m 16s
CI (ubuntu-22.04, 6.4, GCC)	3m 6s
CI (ubuntu-22.04, 5.15, e2ee)	6m 18s
CI (macos-11, 6.4, e2ee)	4m 21s
CI (macos-11, 6.4)	1m 0s
CI (macos-11, 5.15, e2ee)	4m 2s
CI (windows-2019, 6.4, e2ee)	8m 46s
CI (windows-2019, 6.4)	3m 12s
CI (windows-2019, 5.15, e2ee)	10m 34s
CI (ubuntu-22.04, 5.15, e2ee, update-api, codeql)	8m 2s
CI (ubuntu-22.04, 5.15, GCC, update-api)	3m 19s
	1h 8m 32s

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