[libc++] Rewrite the IWYU generation (#78295)

This simplifies the IWYU generation script by treating everything as a
file, instead of dealing with directories and files separately.

This has the downside that the `libcxx.imp` file is a lot larger than it
used to be, however we now have the flexibility of mapping files under
detail directories to different public headers. For example, this allows
us to map <__fwd/subrange.h> to <ranges> but <__fwd/pair.h> to
<utility>.

This patch also adds basic validation to ensure that we never map a
header to a public header that doesn't exist. We may still be missing
some mappings or we may be mapping to incorrect headers, but we won't be
mapping to headers that downright don't exist.

Fixes #63346

Author: ldionne