Windows bots fail due to preprocessor issue

[alan-baker]

For example (from #1014):

C:\Program Files (x86)\Windows Kits\10\Include\10.0.14393.0\um\oaidl.h(473,17): warning C5103: pasting '/' and '/' does not result in a valid preprocessing token (compiling source file T:\src\github\clspv\third_party\llvm\llvm\lib\Support\CrashRecoveryContext.cpp) [T:\src\github\clspv\build\third_party\llvm\lib\Support\LLVMSupport.vcxproj]
C:\Program Files (x86)\Windows Kits\10\Include\10.0.14393.0\shared\wtypes.h(745): message : in expansion of macro '_VARIANT_BOOL' (compiling source file T:\src\github\clspv\third_party\llvm\llvm\lib\Support\CrashRecoveryContext.cpp) [T:\src\github\clspv\build\third_party\llvm\lib\Support\LLVMSupport.vcxproj]
  ELFAttributeParser.cpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.14393.0\um\oaidl.h(473,17): error C2059: syntax error: '/' (compiling source file T:\src\github\clspv\third_party\llvm\llvm\lib\Support\CrashRecoveryContext.cpp) [T:\src\github\clspv\build\third_party\llvm\lib\Support\LLVMSupport.vcxproj]

Need to further investigate whether the bot can be upgraded or other ways to fix the problem.