From the command line, the debug options "stack", with -gVS enabling all
of generateDebugInfo, emitNonSemanticShaderDebugInfo and
emitNonSemanticShaderDebugSource, however the programmatic interface
allows setting the latter options without the former. In this case, the
string corresponding to the source filename never gets emitted and some
debuginfo instructions end up with zero ID operands, resulting in
invalid SPIR-V.
Fixes#3240
