Fix compilation with clang on Windows. #1635

Job	Run time
clang-format	12s
Windows VS-2019	1m 10s
Windows VS-2019 (clangcl)	1m 8s
Windows VS-latest	49s
Windows VS-latest (clangcl)	42s
Linux clang10	3m 15s
Linux clang11	2m 13s
Linux clang12	2m 50s
Linux gcc9	2m 49s
Linux gcc10	3m 30s
Linux gcc10 unity	2m 30s
Linux gcc10 ninja in-source	2m 35s
Linux gcc10 ninja out-of-source	3m 5s
Linux gcc10 ninja-unity	1m 58s
macOS xcode-10.3	1s
macOS xcode-11.7	1s
macOS xcode-12.4	1s
macOS xcode-12.4-ninja in-source	1s
macOS xcode-12.4-ninja out-of-source	1s
	28m 51s

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