Clean up LSDA name generation and use for SJLJ exception handling. This

makes an eggregious hack somewhat more palatable. Bringing the LSDA forward
and making it a GV available for reference would be even better, but is
beyond the scope of what I'm looking to solve at this point.

Objective C++ code could generate function names that broke the previous
scheme. This fixes that.

llvm-svn: 80649