[Support] Work around an issue when building with old versions of libstdc++

llvm/lib/Support/UnicodeNameToCodepoint.cpp:189:12: error: chosen constructor is explicit in copy-initialization
    return {N, false, 0};
           ^~~~~~~~~~~~~
/usr/include/c++/5.4.0/tuple:479:19: note: explicit constructor declared here
        constexpr tuple(_UElements&&... __elements)
                  ^