Louis Dionne 924ea3bb53 [libc++] NFC: Move unwrap_iter to its own header
This re-applies 9968896cd62a, which was reverted in b13edf6e907b because
it broke the build.

Differential Revision: https://reviews.llvm.org/D103369
2021-06-01 21:46:59 -04:00
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