lldb support: under debugger support flag, when sending message

to forward class, and assigning to an 'id' type var, message
sends default to 'id'. // rdar"//10988847



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152420 91177308-0d34-0410-b5e6-96231b3b80d8