[objcmt] When checking whether the subscripting methods are declared use

ObjCInterfaceDec::lookupInstanceMethod to make sure we check categories as well
and update related tests.

rdar://11695288

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158697 91177308-0d34-0410-b5e6-96231b3b80d8