Make sure that "add-dsym" can't crash us when using it.

I am fixing this by:
1 - make sure we aren't trying to set the symbol file for a module to the same thing it already has and leaving it alone if it is the same
2 - keep all old symbol files around in the module in case there are any outstanding type references

<rdar://problem/18029116>

llvm-svn: 233757