implement defining C variadic functions

test/cases/compile_errors/invalid_variadic_function.zig

@@ -0,0 +1,12 @@
+fn foo(...) void {}
+fn bar(a: anytype, ...) callconv(a) void {}
+
+comptime { _ = foo; }
+comptime { _ = bar; }
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:1: error: variadic function must have 'C' calling convention
+// :2:1: error: generic function cannot be variadic