fix: un-qualify Range::value_type for compatibility with fmt v11

[mhx]

folly::StringPiece no longer works with fmt v11 due to value_type being const char instead of the char expected by fmt.

Looking at std::span, which is probably the closest equivalent to folly::Range, value_type is defined as std::remove_cv_t<T>, so I think it would make sense for folly::Range to define value_type similarly, i.e. without cv-qualification.

This change removes cv-qualifiers from value_type and replaces the the use of value_type& by reference and const value_type& by the newly introduced const_reference.

See https://github.com/fmtlib/fmt/pull/4086 for additional context.

[facebook-github-bot]

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[michel-slm]

Thanks! I'll add this to the Fedora build while you work with @yfeldblum to get this merged upstream

[vitaut]

Superseded by https://github.com/facebook/folly/commit/21e8dcd464ee46b2144a1e4d4c0e452355ae15f0 but thanks for working on this!