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

[Tom Honermann via cfe-commits]

================
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// expected-warning at +1{{'clang::sycl_external' attribute ignored}}
----------------
tahonermann wrote:

```suggestion
// These tests validate that the sycl_external attribute is ignored when SYCL
// support is not enabled.

// expected-warning at +1{{'clang::sycl_external' attribute ignored}}
```

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