[clang] [Clang] Ensure default arguments in friend declarations are only allowed in defining declarations to prevent multiple reachable declarations (PR #113777)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

> @zyn0217 should all the commits be squashed into one?

Of course.

By saying “flesh out the commit message”, I meant to edit the PR description on github, which would become the final commit message when we merge the PR.

https://github.com/llvm/llvm-project/pull/113777