Matching C++ parser changes from commit 67ca9166d3.

Without these changes the core library parser fails to parse:

 - core/fmt/fmt.odin:

     Line 126 (and others):
       sbprint(&str, ..args, sep=sep)

 - core/fmt/fmt_os.odin:

     Line 17 (and others):
       return wprint(w, ..args, sep=sep)

 - core/log/log.odin:

     Line 79 (and others):
       logf(.Debug,   fmt_str, ..args, location=location)

 - core/runtime/core_builtin.odin:

     Line 475 (and others):
       return append_elems(array, ..args, loc=loc)

core/odin/parser/parser.odin

@@ -2941,9 +2941,9 @@ parse_call_expr :: proc(p: ^Parser, operand: ^ast.Expr) -> ^ast.Expr {
 	p.expr_level += 1
 	open := expect_token(p, .Open_Paren)
 
+	seen_ellipsis := false
 	for p.curr_tok.kind != .Close_Paren &&
-	    p.curr_tok.kind != .EOF &&
-	    ellipsis.pos.line == 0 {
+	    p.curr_tok.kind != .EOF {
 
 		if p.curr_tok.kind == .Comma {
 			error(p, p.curr_tok.pos, "expected an expression not ,")
@@ -2972,10 +2972,16 @@ parse_call_expr :: proc(p: ^Parser, operand: ^ast.Expr) -> ^ast.Expr {
 			fv.value = value
 
 			arg = fv
+		} else if seen_ellipsis {
+			error(p, arg.pos, "Positional arguments are not allowed after '..'")
 		}
 
 		append(&args, arg)
 
+		if ellipsis.pos.line != 0 {
+			seen_ellipsis = true
+		}
+
 		if !allow_token(p, .Comma) {
 			break
 		}