[clangd] Fix the crash in getQualification

Happens when DestContext is LinkageSpecDecl and hense CurContext happens to be
both not TagDecl and NamespaceDecl.

Minimal reproducer: trigger define outline in

```
namespace ns {
extern "C" {
typedef int foo;
}
foo Fo^o(int id) { return id; }
}
```

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D107047

clang-tools-extra/clangd/AST.cpp

@@ -119,14 +119,17 @@ getQualification(ASTContext &Context, const DeclContext *DestContext,
       (void)ReachedNS;
       NNS = NestedNameSpecifier::Create(Context, nullptr, false,
                                         TD->getTypeForDecl());
-    } else {
+    } else if (auto *NSD = llvm::dyn_cast<NamespaceDecl>(CurContext)) {
       ReachedNS = true;
-      auto *NSD = llvm::cast<NamespaceDecl>(CurContext);
       NNS = NestedNameSpecifier::Create(Context, nullptr, NSD);
-      // Anonymous and inline namespace names are not spelled while qualifying a
-      // name, so skip those.
+      // Anonymous and inline namespace names are not spelled while qualifying
+      // a name, so skip those.
       if (NSD->isAnonymousNamespace() || NSD->isInlineNamespace())
         continue;
+    } else {
+      // Other types of contexts cannot be spelled in code, just skip over
+      // them.
+      continue;
     }
     // Stop if this namespace is already visible at DestContext.
     if (IsVisible(NNS))

clang-tools-extra/clangd/unittests/ASTTests.cpp

@@ -259,6 +259,18 @@ TEST(ClangdAST, GetQualification) {
           {"ns2::", "ns2::", ""},
           {"ns1::"},
       },
+      {
+          R"cpp(
+            namespace ns {
+            extern "C" {
+            typedef int Foo;
+            }
+            }
+            void insert(); // ns::Foo
+          )cpp",
+          {"ns::"},
+          {},
+      },
   };
   for (const auto &Case : Cases) {
     Annotations Test(Case.Test);