[llvm] [libLTO] add thinlto caching flags to libLTO (PR #168567)

[Steven Wu via llvm-commits]

https://github.com/cachemeifyoucan commented:

I am not a big fan of adding this if this is only used for one year but I am also not strongly against it.

Please add a test to exercise the options. 

https://github.com/llvm/llvm-project/pull/168567