What is clang error code 254?

[bob prohaska]

The pattern of clang crashes during buildworld on RPI3 continues, 
most consistently reporting Error code 254.

I've looked on the web for clang error codes and find nothing.
Even the llvm.org website isn't helpful. There are references 
to error 254, but nothing seems recent, and no error list is
mentioned.

Am I overlooking something obvious?

Thanks for reading,

bob prohaska