This was missed when mass-adding support for other LTO options in 0b51e648307cf6c21c463d3e73e51c03aaa8c9e2. Group the existing thinlto_cache_dir with these other options in a new group, next to the other LTO options. This skips adding the options --thinlto-emit-index-files and --thinlto-single-module=, which don't seem to have corresponding options on the lld-link level currently. This should fix https://github.com/mstorsjo/llvm-mingw/issues/386.