[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[Tom Honermann via cfe-commits]

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@@ -0,0 +1,70 @@
+// RUN: %clang_cc1 -fsycl-is-device -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsycl-is-device -std=c++20 -fsyntax-only -verify -DCPP20 %s
+// Semantic tests for sycl_external attribute
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tahonermann wrote:

Add spacing to avoid potential merge conflicts or careless additions of additional RUN lines in the future.
```suggestion

// Semantic tests for the sycl_external attribute.
```

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