Objective-C: This patch fixes a none-issuance of warning

when result type of protocol property and getter method
differ by fixing a more serious problem. When a forward
protocol declaration comes between its definition and
its use in class protocol list, the forward protocol
ast was being used in building the protocol list.
// rdar://12522752


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179108 91177308-0d34-0410-b5e6-96231b3b80d8