Fix warning about get_unchecked returning a C-incompatible reference
Fix warning about get_unchecked returning a C-incompatible reference

    out/tests/ffi/lib.rs.cc:1011:22: warning: 'cxxbridge03$std$vector$tests$Shared$get_unchecked' has C-linkage specified, but returns user-defined type 'const tests::Shared &' which is incompatible with C [-Wreturn-type-c-linkage]
    const tests::Shared &cxxbridge03$std$vector$tests$Shared$get_unchecked(const ::std::vector<tests::Shared> &s, size_t pos) noexcept {
                         ^
    out/tests/ffi/lib.rs.cc:1038:17: warning: 'cxxbridge03$std$vector$tests$C$get_unchecked' has C-linkage specified, but returns user-defined type 'const tests::C &' which is incompatible with C [-Wreturn-type-c-linkage]
    const tests::C &cxxbridge03$std$vector$tests$C$get_unchecked(const ::std::vector<tests::C> &s, size_t pos) noexcept {
                    ^
    2 warnings generated.

dtolnay/cxx,dtolnay/cxx,dtolnay/cxx,dtolnay/cxx