Prevent foo.bar = {} when bar is a flexible array member.

test/test_suite/struct/flex_more_errors.c3

@@ -0,0 +1,25 @@
+module main;
+
+struct OwnedString
+{
+	usz len;
+	char[*] data;
+}
+
+fn void test1()
+{
+	OwnedString owned;
+	owned.data = {}; // #error: You can't assign to a flexible array member
+}
+
+fn void test2()
+{
+	$typefrom(OwnedString.data.typeid) x = 1; // #error:  'char[*]' has unknown size, and
+}
+
+fn int main()
+{
+	OwnedString owned;
+	owned.data[0] = 1;
+    return 0;
+}