[libc++] Rename __tuple to __tuple_dir to avoid file collision

Rename the `__tuple` directory in libc++ headers to `__tuple_dir`
to avoid file collision when installing.  Historically, `__tuple` has
been a file and it has been replaced by a directory
in 2d52c6bf.  Replacing a regular file
with a directory (or more importantly, the other way around when
downgrading) is not universally supported.  Since this is an internal
header, its actual name should not matter, so just rename it to avoid
problems.

Differential Revision: https://reviews.llvm.org/D139270