Don't dump llvm-config --cmakedir output if command fails.

This patch adjusts the out-of-tree CMake configuration so that
the stderr output is ignored when an old llvm-config is found
that doesn't support --cmakedir.

llvm-svn: 291991

libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake

@@ -43,7 +43,8 @@ macro(find_llvm_parts)
     execute_process(
       COMMAND ${LLVM_CONFIG_PATH} --cmakedir
       RESULT_VARIABLE HAD_ERROR
-      OUTPUT_VARIABLE CONFIG_OUTPUT)
+      OUTPUT_VARIABLE CONFIG_OUTPUT
+      ERROR_QUIET)
     if(NOT HAD_ERROR)
       string(STRIP "${CONFIG_OUTPUT}" LLVM_CMAKE_PATH)
     else()