Add test cases for false positives on -Wnull-arithmetic from Richard

Add test cases for false positives on -Wnull-arithmetic from Richard
Trieu, and fix them by checking for array and function types as well as
pointer types.

I've added a predicate method on Type to bundle together the logic we're
using here: isPointerLikeType(). I'd welcome better names for this
predicate, this is the best I came up with. It's implemented as a switch
to be a touch lighter weight than all the chained isa<...> casts that
would result otherwise.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133383 91177308-0d34-0410-b5e6-96231b3b80d8