Pass -Wno-unused-command-line-argument to clang in `is_flag_supported… #879

Job	Run time
MSRV (ubuntu-latest)	4m 55s
MSRV (windows-latest)	7m 4s
Test CUDA support	53s
Rustfmt	3s
Test (stable)	39s
Test (beta)	44s
Test (nightly)	42s
Test (linux32)	1m 1s
Test (macos)	1m 3s
Test (aarch64-ios)	57s
Test (win32)	1m 14s
Test (win64)	1m 31s
Test (mingw32)	1m 51s
Test (mingw64)	2m 7s
Test (windows-2019)	2m 46s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 19s
	29m 49s

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