Patch for va_arg and references

[Mark Mitchell]

Geoff --

  Your recent patch to c-common.c breaks these test cases:

FAIL: g++.jason/thunk3.C (test for excess errors)
FAIL: g++.other/vaarg1.C (test for excess errors)
FAIL: g++.other/vaarg2.C (test for excess errors)
FAIL: g++.pt/vaarg.C (test for excess errors)

  on i686-pc-linux-gnu.

  The reason is that `&ap' is not an lvalue in C++ (even though `ap'
is), and references must be bound to lvalues.  Effectively, we've now
got:

  extern void va_start (void *&);
  void *ap;
  va_start (&ap);

which isn't legal C++.

-- Mark