[clang] [RFC][clang] Handle friend function that was a definition but became only a declaration during AST deserialization (PR #132214)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

LGTM generally.

Some other personal suggestion to the title, it might be better to add `[modules]` to the title. Since my mail filter will highlight these mails with the tag : ) This might be why I missed you last ping.

https://github.com/llvm/llvm-project/pull/132214