a08506e374
This was missed when mass-adding support for other LTO options in
0b51e64830.
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.