Fix <rdar://problem/6210791> clang ObjC rewriter: @try / @catch block with no @finall...

Fix <rdar://problem/6210791> clang ObjC rewriter: @try / @catch block with no @finally does not call objc_exception_try_exit.

Need a couple tweaks to RewriteObjCTryStmt(). Need to deal with implicit finally clauses (to make sure objc_exception_try_exit is called). Also fixed a related bug where we need to generate an implicit @catch else clause (to again make sure objc_exception_try_exit is called).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@56104 91177308-0d34-0410-b5e6-96231b3b80d8