[runtimes] Remove support for standalone builds

Standalone build have been deprecated for some time now, so this
commit removes support for those builds entirely from libc++, libc++abi
and libunwind.

This, along with the removal of other legacy ways to build, will allow
for major build system simplifications.

Differential Revision: https://reviews.llvm.org/D119255

NOKEYCHECK=True
GitOrigin-RevId: 6f17768e11480063f4c2bcbeea559505fee3ea19
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 390c149..aabecc5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -27,6 +27,9 @@
 set(LIBCXX_BINARY_INCLUDE_DIR "${LIBCXX_BINARY_DIR}/include/c++build")
 
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBCXX_STANDALONE_BUILD)
+  message(FATAL_ERROR "The Standalone build has been deprecated since LLVM 14, and it is not supported anymore. "
+                      "Please use one of the ways described at https://libcxx.llvm.org/BuildingLibcxx.html for "
+                      "building libc++.")
   project(libcxx CXX C)
 
   set(PACKAGE_NAME libcxx)
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 9bb06cf..0bf24b9 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -57,3 +57,7 @@
 
 Build System Changes
 --------------------
+
+- Support for standalone builds have been entirely removed from libc++, libc++abi and
+  libunwind. Please use :ref:`these instructions <build instructions>` for building
+  libc++, libc++abi and/or libunwind.
diff --git a/utils/ci/buildkite-pipeline.yml b/utils/ci/buildkite-pipeline.yml
index 0d8f6c8..ae4c07a 100644
--- a/utils/ci/buildkite-pipeline.yml
+++ b/utils/ci/buildkite-pipeline.yml
@@ -297,19 +297,6 @@
             limit: 2
       timeout_in_minutes: 120
 
-    - label: "Legacy standalone build"
-      command: "libcxx/utils/ci/run-buildbot legacy-standalone"
-      artifact_paths:
-        - "**/test-results.xml"
-      agents:
-        queue: "libcxx-builders"
-        os: "linux"
-      retry:
-        automatic:
-          - exit_status: -1  # Agent was lost
-            limit: 2
-      timeout_in_minutes: 120
-
     - label: "Legacy LLVM_ENABLE_PROJECTS build"
       command: "libcxx/utils/ci/run-buildbot legacy-project-build"
       artifact_paths:
diff --git a/utils/ci/run-buildbot b/utils/ci/run-buildbot
index 080c964..b8bebe4 100755
--- a/utils/ci/run-buildbot
+++ b/utils/ci/run-buildbot
@@ -516,48 +516,6 @@
           -DLIBCXX_CXX_ABI=libcxxabi
     check-runtimes
 ;;
-legacy-standalone)
-    clean
-
-    echo "--- Generating CMake"
-    ${CMAKE} \
-          -S "${MONOREPO_ROOT}/libcxx" \
-          -B "${BUILD_DIR}/libcxx" \
-          -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
-          -DCMAKE_BUILD_TYPE=RelWithDebInfo \
-          -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
-          -DLLVM_PATH="${MONOREPO_ROOT}/llvm" \
-          -DLIBCXX_CXX_ABI=libcxxabi \
-          -DLIBCXX_INCLUDE_BENCHMARKS=OFF \
-          -DLIBCXX_CXX_ABI_INCLUDE_PATHS="${MONOREPO_ROOT}/libcxxabi/include" \
-          -DLIBCXX_CXX_ABI_LIBRARY_PATH="${BUILD_DIR}/libcxxabi/lib"
-
-    ${CMAKE} \
-          -S "${MONOREPO_ROOT}/libcxxabi" \
-          -B "${BUILD_DIR}/libcxxabi" \
-          -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
-          -DCMAKE_BUILD_TYPE=RelWithDebInfo \
-          -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
-          -DLLVM_PATH="${MONOREPO_ROOT}/llvm" \
-          -DLIBCXXABI_LIBCXX_PATH="${MONOREPO_ROOT}/libcxx" \
-          -DLIBCXXABI_LIBCXX_INCLUDES="${BUILD_DIR}/libcxx/include/c++/v1" \
-          -DLIBCXXABI_LIBCXX_LIBRARY_PATH="${BUILD_DIR}/libcxx/lib"
-
-    echo "+++ Generating libc++ headers"
-    ${NINJA} -vC "${BUILD_DIR}/libcxx" generate-cxx-headers
-
-    echo "+++ Building libc++abi"
-    ${NINJA} -vC "${BUILD_DIR}/libcxxabi" cxxabi
-
-    echo "+++ Building libc++"
-    ${NINJA} -vC "${BUILD_DIR}/libcxx" cxx
-
-    echo "+++ Running the libc++ tests"
-    ${NINJA} -vC "${BUILD_DIR}/libcxx" check-cxx
-
-    echo "+++ Running the libc++abi tests"
-    ${NINJA} -vC "${BUILD_DIR}/libcxxabi" check-cxxabi
-;;
 aarch64)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AArch64.cmake" \