[libcxx] [doc] Update Windows build instructions after deprecating the legacy standalone builds

[ldionne]

This is great! However, do we even need to have special instructions for Windows? In an ideal world, we'd be able to share the same instructions for Windows and non-Windows. Until that can be achieved, fixing the Windows-specific documentation is the correct thing to do. Thanks!

[mstorsjo]

In D122715#3416461, @ldionne wrote:

However, do we even need to have special instructions for Windows? In an ideal world, we'd be able to share the same instructions for Windows and non-Windows.

Ideally one wouldn't require special instructions, indeed. But there are a bunch of environmental details about how to set things up etc, so it's probably good to have concrete examples with commentery about things you need to consider.

libcxx/docs/BuildingLibcxx.rst

 CMake + Visual Studio
 ---------------------
 
 Building with Visual Studio currently does not permit running tests. However,
 it is the simplest way to build.
 
 .. code-block:: batch
 
-  > cmake -G "Visual Studio 16 2019" -S libcxx -B build ^
-          -T "ClangCL"                                  ^
-          -DLIBCXX_ENABLE_SHARED=YES                    ^
-          -DLIBCXX_ENABLE_STATIC=NO                     ^
+  > cmake -G "Visual Studio 16 2019" -S runtimes -B build ^
+          -T "ClangCL"                                    ^
+          -DLLVM_ENABLE_RUNTIMES=libcxx                   ^
+          -DLIBCXX_ENABLE_SHARED=YES                      ^
+          -DLIBCXX_ENABLE_STATIC=NO                       ^
           -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
   > cmake --build build
 
 CMake + ninja (MSVC)
 --------------------
 
 Building with ninja is required for development to enable tests.
 A couple of tests require Bash to be available, and a couple dozens

 path. From there, launch an MSYS2 shell via e.g.
 ``C:\msys64\msys2_shell.cmd -full-path -mingw64`` (preserving the earlier
 environment, allowing the MSVC headers/libraries and clang-cl to be found).
 
 In either case, then run:
 
 .. code-block:: batch
 
-  > cmake -G Ninja -S libcxx -B build                                                 ^
+  > cmake -G Ninja -S runtimes -B build                                               ^
           -DCMAKE_C_COMPILER=clang-cl                                                 ^
           -DCMAKE_CXX_COMPILER=clang-cl                                               ^
+          -DLLVM_ENABLE_RUNTIMES=libcxx                                               ^
           -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
   > ninja -C build cxx
   > ninja -C build check-cxx
 
 If you are running in an MSYS2 shell and you have installed the
 MSYS2-provided clang package (which defaults to a non-MSVC target), you
 should add e.g. ``-DCMAKE_CXX_COMPILER_TARGET=x86_64-windows-msvc`` (replacing
 ``x86_64`` with the architecture you're targeting) to the ``cmake`` command
 line above. This will instruct ``check-cxx`` to use the right target triple
 when invoking ``clang++``.
 
 Also note that if not building in Release mode, a failed assert in the tests
 pops up a blocking dialog box, making it hard to run a larger number of tests.
 
 CMake + ninja (MinGW)
 ---------------------
 
 libcxx can also be built in MinGW environments, e.g. with the MinGW
 compilers in MSYS2. This requires clang to be available (installed with
 e.g. the ``mingw-w64-x86_64-clang`` package), together with CMake and ninja.
 
 .. code-block:: bash
 
-  > cmake -G Ninja -S libcxx -B build                                                 \
+  > cmake -G Ninja -S runtimes -B build                                               \
           -DCMAKE_C_COMPILER=clang                                                    \
           -DCMAKE_CXX_COMPILER=clang++                                                \
+          -DLLVM_ENABLE_RUNTIMES=libcxx                                               \
           -DLIBCXX_HAS_WIN32_THREAD_API=ON                                            \
           -DLIBCXX_CXX_ABI=libstdc++                                                  \
           -DLIBCXX_TARGET_INFO="libcxx.test.target_info.MingwLocalTI"
   > ninja -C build cxx
   > cp /mingw64/bin/{libstdc++-6,libgcc_s_seh-1,libwinpthread-1}.dll lib
   > ninja -C build check-cxx
 
 As this build configuration ends up depending on a couple other DLLs that