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

[via cfe-commits]

================
@@ -12251,6 +12264,9 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
   if (NewFD->hasAttr<SYCLKernelEntryPointAttr>())
     SYCL().CheckSYCLEntryPointFunctionDecl(NewFD);
 
+  if (NewFD->hasAttr<SYCLExternalAttr>())
+    SYCL().CheckSYCLExternalFunctionDecl(NewFD);
----------------
schittir wrote:


 ```
 namespace { struct S9 {}; }
 struct T9 {
   using type = S9;
 };
 template<typename>
 [[clang::sycl_external]] void func9() {} // error here : {{'sycl_external' can only be applied to functions with external linkage}}
 template<typename T>
 [[clang::sycl_external]] void test_func9() {
   func9<typename T::type>(); // note here: {{in instantiation of function template specialization 'func9<(anonymous namespace)::S9>' requested here}}
 }
 template void test_func9<T9>(); // note here: {{in instantiation of function template specialization 'test_func9<T9>' requested here}}
 ```

This case is being diagnosed at present. Will fix it. 

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