Simplify generate code for exceptions:

 - There is no need to branch on the rethrow variable to determine if
   we need to call objc_exception_try_exit. We know whether an
   exception was thrown, so just branch to a different target if we
   want to skip the try_exit. 

   This is a slight semantic departure from gcc, but only for throwing
   nil, which is undefined (and for which gcc emits broken code).

 - Also fixes a bug in current code which was calling try_exit too
   many times when an exception was uncaught (but there were some
   handlers).

Fix bug introduced in prev. commit, the type of the @catch parameter
was uninitialized.

llvm-svn: 56754