Fix the ASan fuse-lld.cc test after LLD r280012

With that change, images built with 'lld-link /debug' always have a
debug directory. If no PDB filename was passed on the command line, then
the filename in the executable is empty.

PDB information would never work anyway if the PDB file name is empty,
so go ahead and try DWARF in that case.

llvm-svn: 280410