[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

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@@ -0,0 +1,52 @@
+// RUN: %clang_cc1 -fsycl-is-device -fsyntax-only -verify -DSYCL %s
+// RUN: %clang_cc1 -fsycl-is-host -fsyntax-only -verify -DHOST %s
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tahonermann wrote:

Since SYCL applies to both host and device, let's differentiate by "host" and "device" (with or without "SYCL")
```suggestion
// RUN: %clang_cc1 -fsycl-is-device -fsyntax-only -verify -DDEVICE %s
// RUN: %clang_cc1 -fsycl-is-host -fsyntax-only -verify -DHOST %s
```

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