[clang] [clang][SYCL] Diagnose variadic sycl_external function (PR #216393)

[Tom Honermann via cfe-commits]

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@@ -640,6 +644,8 @@ The following examples demonstrate the use of this attribute:
 [[clang::sycl_external]] static void Quux() { /* ... */ } // error:  Quux() has internal linkage.
 ```
 
+  [[clang::sycl_external]] void Corge(int, ...); // warning: attribute ignored;  Corge() is variadic.
+
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tahonermann wrote:

The new example needs to be moved inside the "```c++" block. Indentation is off as well.

(This was probably correct previously; it probably got messed up upon rebase after the migration of documentation to markdown format).

https://github.com/llvm/llvm-project/pull/216393