[clang] [llvm] [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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@@ -1119,6 +1119,14 @@ Windows Support
 
 LoongArch Support
 ^^^^^^^^^^^^^^^^^
+- Added a code model attribute for the global variable. The following values
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xen0n wrote:

"The `model` attribute is now supported for overriding the default code model used to access global variables." or something like that?

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