[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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@@ -70,6 +70,8 @@ Changes to the LLVM IR
 
 * Added `llvm.exp10` intrinsic.
 
+* Added a code model attribute for the `global variable <LangRef.html#global-variables>`_.
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xen0n wrote:

Refer to the IR construct verbatim, i.e. `code_model`, for better readability?

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