[Sema] Don't crash trying to diagnose abs called on a pointer type

Clang tries to figure out if a call to abs is suspicious by looking
through implicit casts to look at the underlying, implicitly converted
type.
Interestingly, C has implicit conversions from pointer-ish types like
function to less exciting types like int.  This trips up our 'abs'
checker because it doesn't know which variant of 'abs' is appropriate.

Instead, diagnose 'abs' called on function types upfront.  This sort of
thing is highly suspicious and is likely indicative of a missing
pointer dereference/function call/array index operation.

This fixes PR25532.

llvm-svn: 253156