Fix PR4092 by improving error recovery in two ways:

1. In a struct field redefinition, don't mark the struct erroneous.  The
   field is erroneous, but the struct is otherwise well formed.
2. Don't emit diagnostics about functions that are known to be broken already.

Either fix is sufficient to silence the second diagnostic in the example,
but the combination is better :)

llvm-svn: 70371