[clang-tools-extra] [libcxx] Allow string to use SSO in constant evaluation. (PR #66576)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > I don't think I will be able to work on that the discussed Clang diagnostics anytime soon. Unless someone else is volunteering to implement it, that may not be implemented at all.
> > So, I'd like to hear explicit consensus is that folks are generally OK to submit this _without_ such a diagnostic.
> 
> Oh, I might have misread or misunderstood the discussion, but I didn't have in mind that we would block this patch from making progress on having a Clang diagnostic. Did other people in this conversation have that expectation?
> 
> Edit: In other words, I think we should ship this and then adding a warning is a nice-to-have followup to reduce the likelihood of people falling into a portability trap.

I think that's a reasonable path forward. 

https://github.com/llvm/llvm-project/pull/66576