[clang] Fix Lambda Mangling in Namespace-Scope Variable Initializers. (PR #159115)

[Tom Honermann via cfe-commits]

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@@ -356,7 +356,7 @@ Sema::getCurrentMangleNumberContext(const DeclContext *DC) {
       return std::make_tuple(&Context.getManglingNumberContext(DC), nullptr);
     }
 
-    return std::make_tuple(nullptr, nullptr);
+    return std::make_tuple(nullptr, ManglingContextDecl);
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tahonermann wrote:

Github won't let me add a comment in the right place, so anchoring it here...

I think we should update the comments for the function declaration (in `clang/include/clang/Sema/Sema.h`) to explain this behavior. Something like this:
```
  /// Compute the mangling number context for a lambda expression or
  /// block literal that appears in the specified declaration context in
  /// consideration of the current expression evaluation and template
  /// instantiation contexts. If the mangling context requires external linkage,
  /// then a mangling number context is returned in the first tuple
  /// element. If the mangling context is non-normal (specialized for
  /// lambda and block types relative to other entities), the overriding
  /// declaration is returned in the second tuple element.
  ///
  /// \param DC - The DeclContext containing the lambda expression or
  /// block literal.
```

https://github.com/llvm/llvm-project/pull/159115