[llvm] [clang] [docs] Add llvm and clang release notes for the global-var code model attribute (PR #78664)

[WÁNG Xuěruì via cfe-commits]

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@@ -130,6 +132,8 @@ Changes to the Hexagon Backend
 
 Changes to the LoongArch Backend
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+* Respect the code model attribute of global variables when different from
+  their default values.
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xen0n wrote:

"Code model of global variables can now be overridden by means of the newly-added `code_model` LLVM IR attribute."?

https://github.com/llvm/llvm-project/pull/78664