Revert "Fix locating of COMPILER_RT_LIBCXX_PATH"

clang-aarch64-sve-vla crashes in msan
clang-s390x-linux-lnt fails to compile libcxx

This reverts commit 14b2b67cd7d5e439c4bc5c8051f68a5fbb683480.

GitOrigin-RevId: bfe5e81826f4d98eeaa2dac08aa67bd0d7a95514
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7c33797..fc7607e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -672,8 +672,7 @@
     # Don't use libcxx if LLVM_ENABLE_PROJECTS does not enable it.
     set(COMPILER_RT_LIBCXX_PATH ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR})
     set(COMPILER_RT_LIBCXXABI_PATH ${LLVM_EXTERNAL_LIBCXXABI_SOURCE_DIR})
-  endif()
-  if(NOT IS_DIRECTORY ${COMPILER_RT_LIBCXX_PATH})
+  else()
     foreach(path IN ITEMS ${LLVM_MAIN_SRC_DIR}/projects/libcxx
                           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxx
                           ${LLVM_MAIN_SRC_DIR}/../libcxx