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

[via cfe-commits]

================
@@ -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) {}
+
+// The first declaration of a SYCL external function is required to have this attribute.
+int foo(); // expected-note {{previous declaration is here}}
+
+[[clang::sycl_external]] int foo(); // expected-error {{'clang::sycl_external' attribute does not appear on the first declaration}}
+
+// Subsequent declrations of a SYCL external function may optionally specify this attribute.
+[[clang::sycl_external]] int boo();
+
+[[clang::sycl_external]] int boo(); // OK
+
+int boo(); // OK
+
+class C {
+  [[clang::sycl_external]] void member();
+};
+
+[[clang::sycl_external]] int main() // expected-error {{'sycl_external' cannot be applied to the 'main' function}}
+{
+  return 0;
+}
+
+class D {
+  [[clang::sycl_external]] void del() = delete; // expected-error {{'sycl_external' cannot be applied to an explicitly deleted function}}
+};
----------------
schittir wrote:

Surprisingly, this doesn't get diagnosed if it is not a member function! I'll look into it. 

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