Don't set TypePointer::isunsigned to true in the frontend.

The previous solution was problematic because the change was language-
visble via the isUnsigned trait. Now, pointers are simply explicitly
checked for in the relevant places. (Note that there might be cases in
the diff where a direct isunsigned() call would have been appropriate –
I only checked for instances where the type clearly cannot be a pointer,
but chose to go the safe route in replicating existing behavior
otherwise).

Fixes DMD testcase 'traits'.

gen/statements.cpp

@@ -1317,7 +1317,7 @@ void ForeachRangeStatement::toIR(IRState* p)
     lower = DtoLoad(keyval);
     assert(lower->getType() == upper->getType());
     llvm::ICmpInst::Predicate cmpop;
-    if (key->type->isunsigned())
+    if (isLLVMUnsigned(key->type))
     {
         cmpop = (op == TOKforeach)
         ? llvm::ICmpInst::ICMP_ULT