[clang] [LinkerWrapper] Always pass `-flto` if the linker supports it (PR #102972)

[Joel E. Denny via cfe-commits]

jdenny-ornl wrote:

> I guess I should just manually pass the optimization level at `O2` for now.

Would -O3 still pass through?

https://github.com/llvm/llvm-project/pull/102972