[clang] [Clang][attr] Add 'kcfi_salt' attribute (PR #141846)

[Miguel Ojeda via cfe-commits]

================
@@ -3643,6 +3643,64 @@ make the function's CFI jump table canonical. See :ref:`the CFI documentation
   }];
 }
 
+def KCFISaltDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Use ``__attribute__((kcfi_salt("<salt>")))`` on a function declaration, function
+definition, or typedef to help distinguish CFI hashes between functions with
+the same type signature.
+
+Example use:
+
+.. code-block:: c
+
+  // .h file:
+  #define __kcfi_salt __attribute__((kcfi_salt("vogon")))
+
+  // Convenient typedefs to avoid nested declarator syntax.
+  typedef int (*fptr_t)(void); // Non-salted function call.
+  typedef int (*fptr_salted_t)(void) __kcfi_salt;
+
+  struct widget_generator {
+    fptr_t init;
+    fptr_salted_t exec;
+    fptr_t teardown;
+  };
+
+  // 1st .c file:
+  static int internal_init(void) { /* ... */ }
+  static int internal_salted_exec(void) __kcfi_salt { /* ... */ }
+  static int internal_teardown(void) { /* ... */ }
+
+  static struct widget_generator _generator = {
+    .init = internal_init,
+    .exec = internal_salted_exec,
+    .teardown = internal_teardown,
+  }
----------------
ojeda wrote:

```suggestion
  };
```

https://github.com/llvm/llvm-project/pull/141846