[libc++] Unify the benchmarks with the test suite (#101399)

Instead of building the benchmarks separately via CMake and running them
separately from the test suite, this patch merges the benchmarks

[libc++] Unify the benchmarks with the test suite (#101399)

Instead of building the benchmarks separately via CMake and running them
separately from the test suite, this patch merges the benchmarks into
the test suite and handles both uniformly.

As a result:
- It is now possible to run individual benchmarks like we run tests
(e.g. using libcxx-lit), which is a huge quality-of-life improvement.

- The benchmarks will be run under exactly the same configuration as
the rest of the tests, which is a nice simplification. This does
mean that one has to be careful to enable the desired optimization
flags when running benchmarks, but that is easy with e.g.
`libcxx-lit <...> --param optimization=speed`.

- Benchmarks can use the same annotations as the rest of the test
suite, such as `// UNSUPPORTED` & friends.

When running the tests via `check-cxx`, we only compile the benchmarks
because running them would be too time consuming. This introduces a bit
of complexity in the testing setup, and instead it would be better to
allow passing a --dry-run flag to GoogleBenchmark executables, which is
the topic of https://github.com/google/benchmark/issues/1827.

I am not really satisfied with the layering violation of adding the
%{benchmark_flags} substitution to cmake-bridge, however I believe
this can be improved in the future.

show more ...

[libc++] Move the benchmarks under libcxx/test (#99371)

This is an intermediate and fairly mechanical step towards unifying the
benchmarks with the rest of the test suite. Moving this around requir

[libc++] Move the benchmarks under libcxx/test (#99371)

This is an intermediate and fairly mechanical step towards unifying the
benchmarks with the rest of the test suite. Moving this around requires
a few changes, notably making sure we don't throw a wrench into the
discovery process of the normal test suite. This won't be a problem
anymore once benchmarks are taken into account by the test setup out of
the box.

show more ...