[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

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@@ -12926,6 +12929,10 @@ bool ASTContext::DeclMustBeEmitted(const Decl *D) {
 
     // FIXME: Functions declared with SYCL_EXTERNAL are required during
     // device compilation.
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tahonermann wrote:

```suggestion
```

https://github.com/llvm/llvm-project/pull/140282