[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
+
+[[clang::sycl_external]] // expected-error {{'sycl_external' can only be applied to functions with external linkage}}
+static void func1() {}
+
+namespace {
+  [[clang::sycl_external]] // expected-error {{'sycl_external' can only be applied to functions with external linkage}}
+  void func2() {}
+
+  struct UnnX {};
+}
+
+[[clang::sycl_external]] // expected-error {{'sycl_external' can only be applied to functions with external linkage}}
+  void func4(UnnX) {}
+
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tahonermann wrote:

Here is another case. DPC++ does diagnose this one. https://godbolt.org/z/9E9aPn6eq
```
namespace { struct S8 {}; }
template<typename>
void func8();
template<> [[clang::sycl_external]] void func8<S8>() {}
```

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