Cannot compile with gcc < 8.1

[aaronkintel]

The release notes indicate that gcc >= 7.1 is sufficient to compile LLVM, but I get the following error when I try with 7.2.0:

<pre>llvm-project/llvm/include/llvm/ADT/StringRef.h:85:65: error: call to non-constexpr function 'static std::size_t std::char_traits<char>::length(const char_type*)'
         : Data(Str), Length(Str ? std::char_traits<char>::length(Str) : 0) {}
                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Constexpr char_traits<>::length was not implemented until gcc 8.1.

[efriedma-quic]

See b4e9977 . CC @d0k

[d0k]

Do we need to bring the fallback to __builtin_strlen back?

[efriedma-quic]

Ping? Looks like this is still broken.

Falling back to __builtin_strlen would work.