[libc++] Rename the default PSTL backend

The so-called default backend is actually not really a default backend. It's more like a composition-based or one-liner backend. It would make more sense to rename it accordingly.

Suggestions:
- composition
- one-liner
- proxy
- intermediate

My personal preference might be `composition`.

Search the repository for the PSTL "default backend" name and its references, then inspect the surrounding implementation and any related tests. Rename the backend consistently to the selected terminology, including references that expose the old name, and run the relevant libc++ tests to confirm the existing behavior is unchanged.

Written by the indexing model from the issue text.