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

[Matt Arsenault 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.
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arsenm wrote:

Helpful to include link to the LangRef reference 

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