Objective-C++ : Fix bitfield ivars.

This fixes a regression present from 8.x;  It used to be OK
to test for a DECL_INITIAL value to flag that an ivar was a
bitfield (the initial value was the width).  This still works
on C / Objective-C, but no longer on C++.  Replace the test
with DECL_C_BIT_FIELD() which is set for both C and C++.

gcc/objc/ChangeLog:

	* objc-next-runtime-abi-02.c (objc_v2_build_ivar_ref): Test
	DECL_C_BIT_FIELD to detect that an ivar is a bitfield.