[clang] [clang] Preserve `externs` following broken declarations (PR #161641)

[Alejandro Álvarez Ayllón via cfe-commits]

alejandro-alvarez-sonarsource wrote:

> Given the summary explicitly mentions that previously things went missing from the AST, should we not have a test that verifies that this does not happen anymore? CC @erichkeane

That was the point of [this line](https://github.com/alejandro-alvarez-sonarsource/llvm-project/blob/d282344e002f9629512bc95a3ed68013a2ffbb88/clang/test/Parser/recovery-after-expected-unqualified-id.cpp#L9). Before it would complaint about [unknown type Int](https://godbolt.org/z/vn6xEMW6v), but I see that an AST dump would have been more explicit about what is going on.

https://github.com/llvm/llvm-project/pull/161641