Fix two rewriter bugs:

Fix two rewriter bugs:

- For @class, don't generate multiple typedefs.
- Handle the following edge case interface...

@interface NSMiddleSpecifier : NSObject {}

@end

...which was incorrectly being rewritten to...

struct _interface_NSMiddleSpecifier {
        struct _interface_NSObject _NSObject;
};
{}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43582 91177308-0d34-0410-b5e6-96231b3b80d8