[analyzer] Check that an ObjCIvarRefExpr's base is non-null even as an lvalue.

[analyzer] Check that an ObjCIvarRefExpr's base is non-null even as an lvalue.

Like with struct fields, we want to catch cases like this early,
so that we can produce better diagnostics and path notes:

  PointObj *p = nil;
  int *px = &p->_x; // should warn here
  *px = 1;

llvm-svn: 164442