[clang] [Clang] Make enums trivially equality comparable (PR #133587)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

> > The summary should not merely link to an issue but describe the problem as well. Two reasons:
> > ```
> > 1. for folks reading this in git log
> > 
> > 2. The issue may not totally explain what is going on.
> > ```
> > 
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> > I actually did not get it at first b/c the issue talks about _char_ and _std::byte_ it took me a bit to realize oh yes _std::byte_ is an enum. A good summary would have clarified this for reviewers.
> 
> What else would you like? I've said in the title that I'm making enums trivially equality comparable. The bug just happens to be fixed by that - it's not like this fixes just that bug.

Something like 

"`std::equal(std::byte)` currently has sub-optimal codegen due to ... and in order to fix this we need to ... and in the process ... was factored out into a standalone function `EqualityComparisonIsDefaulted` ..." 

https://github.com/llvm/llvm-project/pull/133587