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

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+// RUN: %clang_cc1 -fsycl-is-device -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsycl-is-device -std=c++20 -fsyntax-only -verify -DCPP20 %s
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schittir wrote:

I am assuming that you also intended to have the code guarded with CPP17 in the body. 

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