parser: add helpful error for C style container declarations

```zig
// a.zig
struct Foo {
    a: u32,
};
```

before:
```
a.zig:1:1: error: expected test, comptime, var decl, or container field, found 'struct'
struct Foo {
^
```
after:
```
a.zig:1:8: error: 'struct Foo' is invalid
struct Foo {
       ^
a.zig:1:8: note: to declare a container do 'const Foo = struct'
struct Foo {
       ^
```

1 files changed, 9 insertions, 0 deletions

diff --git a/src/Module.zig b/src/Module.zig
index 3c4962c587..aa5cddd35c 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -3335,6 +3335,15 @@ pub fn astGenFile(mod: *Module, file: *File) !void {
                 .parent_decl_node = 0,
                 .lazy = .{ .byte_abs = token_starts[file.tree.errors[2].token] },
             }, err_msg, "field after declarations here", .{});
+        } else if (parse_err.tag == .c_style_container) {
+            const note = file.tree.errors[1];
+            try mod.errNoteNonLazy(.{
+                .file_scope = file,
+                .parent_decl_node = 0,
+                .lazy = .{ .byte_abs = token_starts[note.token] },
+            }, err_msg, "to declare a container do 'const {s} = {s}'", .{
+                file.tree.tokenSlice(note.token), note.extra.expected_tag.symbol(),
+            });
         }
 
         {