Simplify generate code for exceptions:

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@56754 91177308-0d34-0410-b5e6-96231b3b80d8