In debugger mode, make ObjC message sends to unknown selectors return

In debugger mode, make ObjC message sends to unknown selectors return
__unknown_anytype, and rewrite such message sends correctly.

I had to bite the bullet and actually add a debugger support mode for this
one, which is a bit unfortunate, but there really isn't anything else
I could imagine doing;  this is clearly just debugger-specific behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135051 91177308-0d34-0410-b5e6-96231b3b80d8