[clang] [clang-tools-extra] [llvm] [PGO] Add ability to mark cold functions as optsize/minsize/optnone (PR #69030)

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WenleiHe wrote:

> > > I don't understand, if you're saying the profile is accurate, then those functions are actually cold, so we should be able to mark them as optsize?
> > 
> > 
> > Accurate is not black or white. The current heuristic requires certain level of accuracy to be effective. If you make the heuristics more aggressive (like what this patch is doing), you're raising the requirement of what can be considered accurate, and profile not meeting that new requirement could see regression with new heuristic.
> > Whether a function is cold or not also depends on what is the calling context and how inlining is done. All that makes function level annotation inherently inaccurate when done before inlining. Not that we shouldn't try it, but it's not as clear cut as it appears to be, and we need to be careful.
> 
> It will be more conservative (pre-inlining), so won't cause additional optimization suppression compared with the current PGSO.

Sample PGO profile has inline context, so in the profile, we may have `foo` as cold and `bar->foo` as hot, but if later inliner rejects `bar->foo` inlining, `foo` can be hot. So marking `foo` as cold pre-inline can still be inaccurate (and not conservative). 

In this PR, you have `PGOColdFuncAttr` default to `ColdFuncOpt::Default`. As long as you keep it that way, this PR is fine for sample pgo (it's no-op unless `pgo-cold-func-opt` is used explicitly). 

https://github.com/llvm/llvm-project/pull/69030