AstGen: make comptime fields in packed and extern structs compile errors

test/cases/compile_errors/invalid_comptime_fields.zig

@@ -0,0 +1,21 @@
+const U = union {
+    comptime a: u32 = 1,
+};
+const E = enum {
+    comptime a = 1,
+};
+const P = packed struct {
+    comptime a: u32 = 1,
+};
+const X = extern struct {
+    comptime a: u32 = 1,
+};
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:5: error: union fields cannot be marked comptime
+// :5:5: error: enum fields cannot be marked comptime
+// :8:5: error: packed struct fields cannot be marked comptime
+// :11:5: error: extern struct fields cannot be marked comptime