[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
llvm-project
b147019f - [clang] Preserve `externs` following broken declarations (#161641)
