Replace all comparisons between ObjCInterfaceDecl pointers with calls

to declaresSameEntity(), as a baby step toward tracking forward
declarations of Objective-C classes precisely. Part of
<rdar://problem/10583531>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@146618 91177308-0d34-0410-b5e6-96231b3b80d8