[libc] fix the .yaml files not being tracked

[michaelrj]

Summary

adds the .yaml files clang-tidy generates as byproducts, which means
that they will be updated properly and cleaned by ninja -t clean

libc/cmake/modules/LLVMLibCObjectRules.cmake

add_custom_command(
       # have changed. add_custom_target explicitly runs everytime therefore we
       # use add_custom_command. This function requires an output file and since
       # linting doesn't produce a file, we create a dummy file using a
       # crossplatform touch.
       COMMAND "${CMAKE_COMMAND}" -E touch ${lint_timestamp}
       COMMENT "Linting... ${target_name}"
       DEPENDS clang-tidy ${internal_target_name} ${ADD_ENTRYPOINT_OBJ_SRCS}
       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+      BYPRODUCTS ${CMAKE_CURRENT_BINARY_DIR}/${target_name}.yaml
     )
 
     add_custom_target(${fq_target_name}.__lint__
       DEPENDS ${lint_timestamp})
     add_dependencies(lint-libc ${fq_target_name}.__lint__)
     add_dependencies(${fq_target_name} ${fq_target_name}.__lint__)
   endif()