[patch][pr17624] Consider hidden decls for isUsed checks

[Rafael Espíndola]

Hi Richard,

The attached patch fixes pr17624, but I worry there might be more bugs
like this. Some analysis must ignore hidden decls so that, for
example, we accept

void f4() {
  void g4(int a, int b = 7);
  {
    void g4(int a, int b = 5);
  }
  void g4(int a = 5, int b);
}

but is isUsed really the only check that has to consider the true most
recent decl?

Cheers,
Rafael
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