[libc++][Docs] Update benchmark doc wrt monorepo

[xgupta]

Summary

Seems this section is not updated since we have transited to llvm-project monorepo.
At the start, we build libcxx under monorepo configuration but later try to make the separate configuration for libcxx build
and running benchmark.

[Mordante]

Thanks for working on this!

libcxx/docs/TestingLibcxx.rst

230

Since our build documentation now uses ninja (https://libcxx.llvm.org/BuildingLibcxx.html#bootstrapping-build) I think it would be nice to change these instructions to ninja.

230

I don't mind removing this line, but I think the documentation should make it clear build directory already contains a generated CMake project.

[xgupta]

libcxx/docs/TestingLibcxx.rst

230

But if the person wants to do testing or running the benchmark, it is certain that they have already built the libcxx and that build directory contains a generated CMake project. Sorry if I misunderstood your comment.

231

fine

[xgupta]

address comment

libcxx/docs/TestingLibcxx.rst

 The benchmark tests are not built by default. The benchmarks can be built using
 the ``cxx-benchmarks`` target.
 
 An example build would look like:
 
 .. code-block:: bash
 
   $ cd build
-  $ cmake [options] <path to libcxx sources>
-  $ make cxx-benchmarks
+  $ ninja cxx-benchmarks
 
 This will build all of the benchmarks under ``<libcxx-src>/benchmarks`` to be
 built against the just-built libc++. The compiled tests are output into
-``build/benchmarks``.
+``build/projects/libcxx/benchmarks``.
 
 The benchmarks can also be built against the platforms native standard library
 using the ``-DLIBCXX_BUILD_BENCHMARKS_NATIVE_STDLIB=ON`` CMake option. This
 is useful for comparing the performance of libc++ to other standard libraries.
 The compiled benchmarks are named ``<test>.libcxx.out`` if they test libc++ and
 ``<test>.native.out`` otherwise.
 
 Also See:
 
   * :ref:`Building Libc++ <build instructions>`
   * :ref:`CMake Options`
 
 Running Benchmarks
 ------------------
 
 The benchmarks must be run manually by the user. Currently there is no way
 to run them as part of the build.
 
 For example:
 
 .. code-block:: bash
 
-  $ cd build/benchmarks
-  $ make cxx-benchmarks
+  $ cd build/projects/libcxx/benchmarks
   $ ./algorithms.libcxx.out # Runs all the benchmarks
   $ ./algorithms.libcxx.out --benchmark_filter=BM_Sort.* # Only runs the sort benchmarks
 
 For more information about running benchmarks see `Google Benchmark`_.