swr/rast: Fix addPassesToEmitFile usage with llvm-7.0.

swr/rast: Fix addPassesToEmitFile usage with llvm-7.0.

Fix build error after llvm-7.0svn r332881 ("CodeGen: Add a dwo output
file argument to addPassesToEmitFile and hook it up to dwo output.").

  CXX      rasterizer/jitter/libmesaswr_la-JitManager.lo
rasterizer/jitter/JitManager.cpp:368:93: error: too few arguments to function call, expected at least 4, have 3
        pTarget->addPassesToEmitFile(*pMPasses, filestream, TargetMachine::CGFT_AssemblyFile);
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                        ^

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>