Do not allow attributes beyond a function's last

parameter, even if it is a varargs function.  Do
allow attributes on the varargs part of a call,
but not beyond the last argument.  Only allow
selected attributes to be on the varargs part of
a call (currently only 'byval' is allowed).  The
reasoning here is that most attributes, eg inreg,
simply make no sense here.

llvm-svn: 45887