[InlineFunction] Set arg attrs even if there only are VarArg attrs.

When using the partial inliner, we might have attributes for forwarded
varargs, but the CodeExtractor does not create an empty argument
attribute set for regular arguments in that case, because it does not know
of the additional arguments. So in case we have attributes for VarArgs, we
also have to make sure we create (empty) attributes for all regular arguments.

This fixes PR36210.

llvm-svn: 324197