[ARC][ObjC++] Use ObjC semantic rules for comparisons between a pointer and

[ARC][ObjC++] Use ObjC semantic rules for comparisons between a pointer and
an ObjC object pointer

When ARC is enabled in Objective-C++, comparisons between a pointer and
Objective-C object pointer typically result in errors like this:
"invalid operands to a binary expression". This error message can be quite
confusing as it doesn't provide a solution to the problem, unlike the non-C++
diagnostic: "implicit conversion of Objective-C pointer type 'id' to C pointer
type 'void *' requires a bridged cast" (it also provides fix-its). This commit
forces comparisons between pointers and Objective-C object pointers in ARC to
use the Objective-C semantic rules to ensure that a better diagnostic is
reported.

rdar://31103857

Differential Revision: https://reviews.llvm.org/D31177

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299080 91177308-0d34-0410-b5e6-96231b3b80d8