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

[Alejandro Álvarez Ayllón via cfe-commits]

https://github.com/alejandro-alvarez-sonarsource created https://github.com/llvm/llvm-project/pull/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"` contexts, 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 libraries 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

>From 446f4f79aa07b0bef23cfb5ce8b87226651e12fe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alvarez at sonarsource.com>
Date: Fri, 2 Jun 2023 18:38:24 +0200
Subject: [PATCH] [clang] Preserve `externs` following broken declarations

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

CPP-4478
---
 clang/lib/Parse/ParseDecl.cpp                            | 3 +++
 .../Parser/recovery-after-expected-unqualified-id.cpp    | 9 +++++++++
 2 files changed, 12 insertions(+)
 create mode 100644 clang/test/Parser/recovery-after-expected-unqualified-id.cpp

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 22c01c4e371f3..d6cd7eb8c2c3d 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
diff --git a/clang/test/Parser/recovery-after-expected-unqualified-id.cpp b/clang/test/Parser/recovery-after-expected-unqualified-id.cpp
new file mode 100644
index 0000000000000..8019b46df1e7b
--- /dev/null
+++ b/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