fix rdar://6611778, a redefinition of an interface was causing an

fix rdar://6611778, a redefinition of an interface was causing an
assertion when the ivars and method list was reset into the existing
interface.  To fix this, mark decls as invalid when they are redefined,
and don't insert ivars/methods into invalid decls.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@65340 91177308-0d34-0410-b5e6-96231b3b80d8