(AppVeyor) Clang-Cl 6 itself crashes, so build fails

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FAILED: C:\RosCMakeNinja\bin\clang-cl.exe [...] c:\reactos-cov\subsystems\win\basesrv\dosdev.c

Assertion failed: Val && "isa<> used on a null pointer", file E:\llvm\include\llvm/Support/Casting.h, line 106

Wrote crash dump file "C:\Users\appveyor\AppData\Local\Temp\1\clang-cl.exe-12c82b.dmp"

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0x77757C04 (0x7F3E8000 0x41DAF004 0x00000000 0x00000000), BaseThreadInitThunk() + 0x24 bytes(s)

0x77D2AD8F (0xFFFFFFFF 0x77D100CA 0x00000000 0x00000000), RtlInitializeExceptionChain() + 0x8F bytes(s)

0x77D2AD5A (0x02F37F1D 0x7F3E8000 0x00000000 0x00000000), RtlInitializeExceptionChain() + 0x5A bytes(s)

clang-cl.exe: error: clang frontend command failed due to signal (use -v to see invocation)

clang version 6.0.0 (trunk 316603)

Target: i686-pc-windows-msvc

Thread model: posix

InstalledDir: C:\RosCMakeNinja\bin

clang-cl.exe: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script.

clang-cl.exe: note: diagnostic msg: 

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PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:

Preprocessed source(s) and associated run script(s) are located at:

clang-cl.exe: note: diagnostic msg: C:\Users\appveyor\AppData\Local\Temp\1\dosdev-661d93.sh

clang-cl.exe: note: diagnostic msg: 

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