[libc++] Restore `basic_ios`'s implicit conversion to `bool` in C++03 mode.

efriedma noted that D104682 broke this test case, reduced from SPEC2006.

    #include <istream>
    bool a(std::istream a) {
        return a.getline(0,0) == 0;
    }

We can unbreak it by restoring the conversion to something-convertible-to-bool.
We chose `void*` in order to match libstdc++.

For more ancient history, see PR19460: https://bugs.llvm.org/show_bug.cgi?id=19460

Differential Revision: https://reviews.llvm.org/D107663