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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > I guess I should just manually pass the optimization level at `O2` for now.
> 
> Would -O3 still pass through?

Should, the behavior that was apparently passing was using `-O2`.

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