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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > Updated to pass `-O2` instead, this was the actual behavior that passed, so it's mostly not a functional change.
> 
> So you just changed the tests not to use -O1? Doesn't this patch then represent a regression in what's supported?
>

I wouldn't consider something "supported" if it only worked because we overruled the user's `-O1` flag and replaced it with `-O2`.

> (Rewriting history with force pushes make it harder to quickly spot what's changed since I looked before. Can you not push new commits instead?)

Dealing with multiple commits is a pain, I miss Phabricator's diff based review system. Only the tests changed.

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