[analyzer] Make GRBugReporter::generatePathDiagnostic iterative, not recursive.

The previous generatePathDiagnostic() was intended to be tail-recursive,
restarting and trying again if a report was marked invalid. However:
 (1) this leaked all the cloned visitors, which weren't being deleted, and
 (2) this wasn't actually tail-recursive because some local variables had
     non-trivial destructors.

This was causing us to overflow the stack on inputs with large numbers of
reports in the same equivalence class, such as sqlite3.c. Being iterative
at least prevents us from blowing out the stack, but doesn't solve the
performance issue: suppressing thousands (yes, thousands) of paths in the
same equivalence class is expensive. I'm looking into that now.

<rdar://problem/13423498>

llvm-svn: 177189