[clang] [clang-repl] Allow private type aliases in out-of-line member function return types (PR #178842)

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fogsong233 wrote:

> Lgtm! Can you squash everything in one commit and write a descriptive commit message. Please include the issue we fix so that it can be closed automatically.

I've finished it.

https://github.com/llvm/llvm-project/pull/178842