Revert the removal of LLVM_ENABLE_PROJECTS for libc++ and libc++abi

This commit reverts the following commits:

- 952f90b72b3546d6b6b038d410f07ce520c59b48
- e6a0800532bb409f6d1c62f3698bdd6994a877dc (D132298)
- 176db3b3ab25ff8a9b2405f50ef5a8bd9304a6d5 (D132324)

These commits caused CI instability and need to be reverted in order
to figure things out again. See the discussion in https://llvm.org/D132324
for more information.

NOKEYCHECK=True
GitOrigin-RevId: 342e0ebd0b7409c8808a90cf38dd45ec7dc57671
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9064afb..b8326d0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -131,17 +131,10 @@
   message(FATAL_ERROR "libc++abi must be built as either a shared or static library.")
 endif()
 
+# TODO: Remove this, which shouldn't be necessary since we know we're being built
+#       side-by-side with libc++.
 set(LIBCXXABI_LIBCXX_INCLUDES "" CACHE PATH
     "Specify path to libc++ includes.")
-if (NOT libcxx IN_LIST LLVM_ENABLE_RUNTIMES)
-  if (NOT IS_DIRECTORY ${LIBCXXABI_LIBCXX_INCLUDES})
-    message(FATAL_ERROR
-      "LIBCXXABI_LIBCXX_INCLUDES=${LIBCXXABI_LIBCXX_INCLUDES} is not a valid directory. "
-      "Please provide the path to where the libc++ headers have been installed.")
-  endif()
-  add_library(cxx-headers INTERFACE)
-  target_include_directories(cxx-headers INTERFACE "${LIBCXXABI_LIBCXX_INCLUDES}")
-endif()
 
 set(LIBCXXABI_HERMETIC_STATIC_LIBRARY_DEFAULT OFF)
 if (WIN32)
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 0b82943..0a4541b 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -61,13 +61,9 @@
   list(APPEND LIBCXXABI_TEST_DEPS cxx_external_threads)
 endif()
 
-if(libcxx IN_LIST LLVM_ENABLE_RUNTIMES)
-  list(APPEND LIBCXXABI_TEST_DEPS cxx)
-endif()
-if(libunwind IN_LIST LLVM_ENABLE_RUNTIMES)
-  if (LIBCXXABI_USE_LLVM_UNWINDER AND TARGET unwind)
-    list(APPEND LIBCXXABI_TEST_DEPS unwind)
-  endif()
+list(APPEND LIBCXXABI_TEST_DEPS cxx)
+if (LIBCXXABI_USE_LLVM_UNWINDER AND TARGET unwind)
+  list(APPEND LIBCXXABI_TEST_DEPS unwind)
 endif()
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxxabi configuration.\n# Do not edit!")