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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

> 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.

Oh, thanks for the good catch! Fixed now.

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