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

[Joel E. Denny via cfe-commits]

jdenny-ornl 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`.

Cases that used to compile successfully (even if more optimized than expected) will no longer.  Sounds like a regression.

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