Support for offsetof

GCC implements for both C and C++ a syntactic extension to implement the offsetof macro.

primary:
        "__builtin_offsetof" "(" ``typename`` "," offsetof_member_designator ")"

offsetof_member_designator:
          ``identifier``
        | offsetof_member_designator "." ``identifier``
        | offsetof_member_designator "[" ``expr`` "]"

This extension is sufficient such that

#define offsetof(``type``, ``member``)  __builtin_offsetof (``type``, ``member``)

is a suitable definition of the offsetof macro. In C++, type may be dependent. In either case, member may consist of a single identifier, or a sequence of member accesses and array references.