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'.

dmd2/mtype.h

@@ -597,8 +597,6 @@ struct TypePointer : TypeNext
     MATCH implicitConvTo(Type *to);
     MATCH constConv(Type *to);
     int isscalar();
-    // LDC: pointers are unsigned
-    int isunsigned() { return TRUE; };
     Expression *defaultInit(Loc loc);
     int isZeroInit(Loc loc);
     TypeInfoDeclaration *getTypeInfoDeclaration();