[clang] [libcxx] [Clang] Implement CWG2369 "Ordering between constraints and substitution" (PR #102857)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @cor3ntin Not much, we just need to move the handling of the friend declaration outside of `getTemplateInstantiationArgs()` :)
> 
> Do you see any other outstanding issues? If not, can we go ahead with the status quo so we have sufficient timeframe in clang 20 cycle in case something else goes wrong?

I don't think I'd be a fan of removing the friend declaration outside of `getTemplateInstantiationArgs`.  That is supposed to be the 'single source' of truth for that, and should work on just about any declaration that could potentially be templated.  So I think we need to do the work to improve it isntead.

https://github.com/llvm/llvm-project/pull/102857