[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 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// expected-warning at +1{{'sycl_external' attribute ignored}}
+[[clang::sycl_external]] void bar() {}
+
+// expected-warning at +1{{'sycl_external' attribute ignored}}
+[[clang::sycl_external]] int a;
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tahonermann wrote:

Per my comment on the diagnostic definition, I think invalid uses should elicit errors rather than warnings. This test should then move to `sycl-external-attr.cpp`.

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