[clang] [clang] more useful error message for decomposition declaration missing initializer (PR #127924)

[Aaron Ballman via cfe-commits]

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@@ -679,6 +679,8 @@ Improvements to Clang's diagnostics
 - Diagnostics for the C++11 range-based for statement now report the correct
   iterator type in notes for invalid iterator types.
 
+- Added a clearer diagnostic for uninitialized decomposition declarations.
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AaronBallman wrote:

```suggestion
- Added a clearer diagnostic for uninitialized decomposition declarations. (#GH90107)
```

https://github.com/llvm/llvm-project/pull/127924