Fix duplicated member declarations in structs for C++ backend (#23512)

When forward declaration is used with pragmas `virtual` or `member`, the
declaration in struct is added twice. It happens because of missing
check for `sfWasForwarded` pragma.

Current compiler generates the following C++ code:
```cpp
struct tyObject_Foo__fFO9b6HU7kRnKB9aJA1RApKw {
N_LIB_PRIVATE N_NOCONV(void, abc)(NI x_p1);
N_LIB_PRIVATE N_NOCONV(virtual void, def)(NI y_p1);
N_LIB_PRIVATE N_NOCONV(void, abc)(NI x_p1);
N_LIB_PRIVATE N_NOCONV(virtual void, def)(NI y_p1);
};
```

0 files changed, 0 insertions, 0 deletions