DebugInfo: Explain why DW_LLE_(GNU_)startx_length is used

This isn't the most object-size efficient encoding, but it's the only
one GDB supports for the pre-standard fission format. I've written fixes
for this twice now... - so perhaps this comment will help me remember
why neither of these have been committed and why I shouldn't try to
write a third fix another year from now...

llvm-svn: 345326