Skip both character pointers and void pointers when diagnosing bad

Skip both character pointers and void pointers when diagnosing bad
argument types for mem{set,cpy,move}. Character pointers, much like void
pointers, often point to generic "memory", so trying to check whether
they match the type of the argument to 'sizeof' (or other checks) is
unproductive and often results in false positives.

Nico, please review; does this miss any of the bugs you were trying to
find with this warning? The array test case you had should be caught by
the array-specific sizeof warning I think.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133136 91177308-0d34-0410-b5e6-96231b3b80d8