Rewriter: Handle unknown write targets #17135

Job	Run time
Arch Linux	20m 18s
CUDA (on Arch)	7m 5s
Arch / PyPy	19m 53s
Fedora	17m 36s
Ubuntu Rolling (gcc, g++)	19m 18s
Gentoo	21m 55s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 16s
OpenSUSE	16m 6s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 54s
Ubuntu Bionic	14m 39s
Ubuntu Rolling (clang, clang++)	22m 8s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 8s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 24s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 28s
	3h 48m 8s

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