[libc++] Re-apply "Always build c++experimental.a""

This re-applies bb939931a1ad, which had been reverted by 09cebfb978de
because it broke Chromium. The issues seen by Chromium should be
addressed by 1d0f79558ca4.

Differential Revision: https://reviews.llvm.org/D128927

NOKEYCHECK=True
GitOrigin-RevId: 7300a651f5f3b43f9fa3a2d9927322b0e061fcb2
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 5069c7f..7c347c6 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -99,8 +99,7 @@
           -T "ClangCL"                                    ^
           -DLLVM_ENABLE_RUNTIMES=libcxx                   ^
           -DLIBCXX_ENABLE_SHARED=YES                      ^
-          -DLIBCXX_ENABLE_STATIC=NO                       ^
-          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
+          -DLIBCXX_ENABLE_STATIC=NO
   > cmake --build build
 
 CMake + ninja (MSVC)
@@ -131,8 +130,7 @@
   > 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
+          -DLLVM_ENABLE_RUNTIMES=libcxx
   > ninja -C build cxx
   > ninja -C build check-cxx
 
@@ -298,23 +296,6 @@
   Path where target-specific libc++ headers should be installed. If a relative
   path, relative to ``CMAKE_INSTALL_PREFIX``.
 
-.. _libc++experimental options:
-
-libc++experimental Specific Options
-------------------------------------
-
-.. option:: LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY:BOOL
-
-  **Default**: ``ON``
-
-  Build and test libc++experimental.a.
-
-.. option:: LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY:BOOL
-
-  **Default**: ``LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY AND LIBCXX_INSTALL_LIBRARY``
-
-  Install libc++experimental.a alongside libc++.
-
 
 .. _ABI Library Specific Options: