Accept CMAKE_SYSTEM_NAME "Windows" for clang (MSYS2 CLANG64 support) #963

Job	Run time
cpp-lib-build (macos-11, clang, ON)	5m 57s
cpp-lib-build (macos-11, clang, OFF)	5m 52s
build (macos-11, tool)	1m 42s
cpp-lib-build (macos-11, gcc, ON)	5m 42s
build (macos-11, cpp)	30m 18s
cpp-lib-build (macos-11, gcc, OFF)	5m 36s
build (macos-11, csharp)	9m 5s
cpp-lib-build (ubuntu-20.04, clang, ON)	1m 13s
build (macos-11, dart)	5m 14s
cpp-lib-build (ubuntu-20.04, clang, OFF)	1m 6s
build (macos-11, go)	7m 27s
cpp-lib-build (ubuntu-20.04, gcc, ON)	3m 0s
build (macos-11, java)	2m 2s
cpp-lib-build (ubuntu-20.04, gcc, OFF)	2m 23s
build (macos-11, javascript)	2m 32s
cpp-lib-build (windows-2022, clang, ON)	4m 7s
build (macos-11, typescript)	18m 7s
cpp-lib-build (windows-2022, clang, OFF)	5m 56s
build (macos-11, php)	13m 11s
cpp-lib-build (windows-2022, cl)	5m 7s
build (macos-11, python3)	1m 58s
build (ubuntu-20.04, tool)	1m 4s
build (ubuntu-20.04, cpp)	15m 16s
build (ubuntu-20.04, csharp)	6m 5s
build (ubuntu-20.04, dart)	3m 34s
build (ubuntu-20.04, go)	2m 13s
build (ubuntu-20.04, java)	1m 23s
build (ubuntu-20.04, javascript)	1m 38s
build (ubuntu-20.04, typescript)	9m 55s
build (ubuntu-20.04, php)	1m 38s
build (ubuntu-20.04, python3)	1m 2s
build (windows-2022, tool)	1m 50s
build (windows-2022, cpp)	21m 47s
build (windows-2022, csharp)	8m 57s
build (windows-2022, dart)	5m 11s
build (windows-2022, go)	5m 41s
build (windows-2022, java)	3m 5s
build (windows-2022, javascript)	2m 11s
build (windows-2022, typescript)	28m 55s
build (windows-2022, php)	2m 7s
build (windows-2022, python3)	2m 8s
	4h 23m 15s

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