[libc++] Fix LWG 4265: std::midpoint should not accept const bool (#174579)

Fixes: #171324

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Co-authored-by: Hristo Hristov <hghristov.rmm@gmail.com>
NOKEYCHECK=True
GitOrigin-RevId: bfd139dacd52e4b2b1af22666846f4e3c5b52a02