From the stack traces, it sounds like clang-cl got blocked writing to stdout because msvc_depends.pl wasn’t reading from the pipe. The first stack trace shows the driver clang-cl process waiting on the child -cc1 invocation, and the second one shows a blocking write syscall. I don’t think there’s any action for us to take here.
If your build system uses configure and make, you might have better luck using the clang driver instead. It can also generate MSVC compatible object files.