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

[Younan Zhang via cfe-commits]

zyn0217 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?

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