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

Treat them as namespaces: if they are at the beginning of the line, they
are likely a good recovery point.

For instance, in

```cpp
1.3.0

extern "C" {
    extern int foo();

    extern "C++" {
        namespace bar {
            void baz();
        };
    }
}

namespace {}
```

Everything until `namespace`... is gone from the AST. Headers (like
libc's C++ `math.h`) can be included from an `extern "C"` context, and
they do an `extern "C++"` back again before including C++ headers (like
`__type_traits`).
However, a malformed declaration just before the include (as the orphan
`1.3.0` in the example) causes everything from these standard headers to
go missing. This patch updates the heuristic to try to recover from the
first `extern` keyword seen, pretty much as it is done for `namespace`.

CPP-4478

0 files changed, 0 insertions, 0 deletions