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