Add implicit conversions for Objective-C qualified ids, e.g.,

Add implicit conversions for Objective-C qualified ids, e.g.,

  id<P0>

The intended overloading behavior of these entities isn't entirely
clear, and GCC seems to have some strange limitations (e.g., the
inability to overload on id<P0> vs. id<P1>). We'll want to revisit
these semantics and determine just how Objective-C++ overloading
should really work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60142 91177308-0d34-0410-b5e6-96231b3b80d8