Apply libc++ fix for <ranges> not compiling at all

Merge commit 44cdca37c01a from llvm git (by Arthur O'Dwyer):

  [libc++] Define `namespace views` in its own detail header.

  Discovered in the comments on D118748: we would like this namespace
  to exist anytime Ranges exists, regardless of whether concepts syntax
  is supported. Also, we'd like to fully granularize the <ranges> header,
  which means not putting any loose declarations at the top level.

  Differential Revision: https://reviews.llvm.org/D118809

(cherry picked from commit 53683b95ef66a12337999587cd98302b1b425920)

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