[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

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 22c01c4..d6cd7eb 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2083,6 +2083,9 @@ void Parser::SkipMalformedDecl() {
         return;
       break;
 
+    case tok::kw_extern:
+      // 'extern' at the start of a line is almost certainly a good
+      // place to pick back up parsing
     case tok::kw_namespace:
       // 'namespace' at the start of a line is almost certainly a good
       // place to pick back up parsing, except in an Objective-C