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 14b2b67.

compiler-rt/CMakeLists.txt

@@ -672,8 +672,7 @@ if(COMPILER_RT_USE_LIBCXX)
     # 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