[clang] [ASTImporter] Fix infinite recurse on return type declared inside body (#68775) (PR #89096)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

I think it would be a good idea to double check this for performance regressions, since this case will recurse into the function every time with this patch.
Though I don't know if there is a better way to test it than to merge it and wait for it to be picked up by google.

Another idea to avoid any potential regressions, is to mark the return type as deduced anyway, with the distinction that 'auto' was not written.

https://github.com/llvm/llvm-project/pull/89096