[libc++] Overhaul the PSTL dispatching mechanism (#88131)

The experimental PSTL's current dispatching mechanism was designed with
flexibility in mind. However, while reviewing the in-progress OpenMP
backend, I realized that the dispatching mechanism based on ADL and
default definitions in the frontend had several downsides. To name a
few:

1. The dispatching of an algorithm to the back-end and its default
   implementation is bundled together via `_LIBCPP_PSTL_CUSTOMIZATION_POINT`.
   This makes the dispatching really confusing and leads to annoyances
   such as variable shadowing and weird lambda captures in the front-end.
2. The distinction between back-end functions and front-end algorithms
   is not as clear as it could be, which led us to call one where we meant
   the other in a few cases. This is bad due to the exception requirements
   of the PSTL: calling a front-end algorithm inside the implementation of
   a back-end is incorrect for exception-safety.
3. There are two levels of back-end dispatching in the PSTL, which treat
   CPU backends as a special case. This was confusing and not as flexible
   as we'd like. For example, there was no straightforward way to dispatch
   all uses of `unseq` to a specific back-end from the OpenMP backend,
   or for CPU backends to fall back on each other.

This patch rewrites the backend dispatching mechanism to solve these
problems, but doesn't touch any of the actual implementation of
algorithms. Specifically, this rewrite has the following
characteristics:

- There is a single level of backend dispatching, however partial backends can
  be stacked to provide a full implementation of the PSTL. The two-level dispatching
  that was used for CPU-based backends is handled by providing CPU-based basis 
  operations as simple helpers that can easily be reused when defining any PSTL 
  backend.

- The default definitions for algorithms are separated from their dispatching logic.

- The front-end is thus simplified a whole lot and made very consistent
  for all algorithms, which makes it easier to audit the front-end for
  things like exception-correctness, appropriate forwarding, etc.

Fixes #70718

1 files changed, 1 insertions, 0 deletions

diff --git a/libcxx/benchmarks/algorithms/pstl.stable_sort.bench.cpp b/libcxx/benchmarks/algorithms/pstl.stable_sort.bench.cpp
index 9357b87..72541f7 100644
--- a/libcxx/benchmarks/algorithms/pstl.stable_sort.bench.cpp
+++ b/libcxx/benchmarks/algorithms/pstl.stable_sort.bench.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include <algorithm>
+#include <execution>
 
 #include "common.h"