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
53683b95ef