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

[Wael Yehia via llvm-commits]

w2yehia wrote:

Thanks Steven (@cachemeifyoucan ) for the review

> Please add a test to exercise the options.

Since these options are added to lto.cpp which is only used to build libLTO.so, I don't think we test libLTO.so in LIT, do we?

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