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);
};
```

tests/cpp/tmember_forward_declaration.nim

@@ -0,0 +1,27 @@
+discard """
+  targets: "cpp"
+  cmd: "nim cpp $file"
+  output: '''
+abc called
+def called
+abc called
+'''
+"""
+
+type Foo = object
+
+proc abc(this: Foo, x: int): void {.member: "$1('2 #2)".}
+proc def(this: Foo, y: int): void {.virtual: "$1('2 #2)".}
+
+proc abc(this: Foo, x: int): void =
+  echo "abc called"
+  if x > 0:
+    this.def(x - 1)
+
+proc def(this: Foo, y: int): void =
+  echo "def called"
+  this.abc(y)
+
+var x = Foo()
+x.abc(1)
+