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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

LGTM.

But I didn't recognize you didn't update `MultiplexExternalSemaSource`. Every time we add new interface to `ExternalASTSource`, we need to update `MultiplexExternalSemaSource` too.

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