[clang] Preserve externs following broken declarations (llvm#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

Author: alejandro-alvarez-sonarsource

clang/docs/ReleaseNotes.rst

@@ -446,6 +446,7 @@ Bug Fixes to AST Handling
   legal representation. This is fixed because ElaboratedTypes don't exist anymore. (#GH43179) (#GH68670) (#GH92757)
 - Fix unrecognized html tag causing undesirable comment lexing (#GH152944)
 - Fix comment lexing of special command names (#GH152943)
+- Use `extern` as a hint to continue parsing when recovering from a malformed declaration.
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^

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

clang/test/Parser/recovery-after-expected-unqualified-id.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -verify %s
+
+3.2 // expected-error {{expected unqualified-id}}
+
+extern "C" {
+    typedef int Int;
+}
+
+Int foo(); // Ok