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

[Wael Yehia via llvm-commits]

w2yehia wrote:

> Can you explain why you can't add flags to linker to call the API that sets the cache directly but want to rely on cl:opt?

Technically we can teach the platform linker to call the API, but users will only get it in the next OS release which is at least a year away. We have users on existing in-service OS releases that we want to provide this functionality to, and they are stuck with the platform linker they have.

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