[Issue 22705] importC: forward reference to struct's typedef gives "struct already exists"

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22705

--- Comment #1 from duser at airmail.cc ---
slightly different example:

```
struct data1_s;

struct data2_s {
        int x;
};
typedef struct data2_s data2_t;

struct data1_s {
    data2_t* p;
};
```

test.c(3): Error: struct `test.data2_s` already exists at test.c(6). Perhaps in
another function with the same name?

the error disappears if the forward declaration of struct data1_s is removed

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