Do not allow attributes beyond a function's last

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45887 91177308-0d34-0410-b5e6-96231b3b80d8