Re-instate contextual conversion to Objective-C pointers in message sends.

When performing an Objective-C message send to a value of class type,
perform a contextual conversion to an Objective-C pointer type. We've
had this for a long time, but it recently regressed. Fixes
<rdar://problem/15234703>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194224 91177308-0d34-0410-b5e6-96231b3b80d8