[SYCL] Add diagnostic test for global_variable_allowed attribute (#6777)

Add cases for this attribute independent of device_global

Author: schittir

clang/test/SemaSYCL/global_variable_allowed.cpp

@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -fsycl-is-device -internal-isystem %S/Inputs -std=c++17 -sycl-std=2020 -verify -fsyntax-only %s
+
+#include "sycl.hpp"
+
+// This test shows that global_variable_allowed attribute allows
+// global variables of type decorated with it to be referenced in device code.
+
+template <typename T>
+struct [[__sycl_detail__::global_variable_allowed]] global_variable_allowed {
+public:
+  const T &get() const noexcept { return *Data; }
+  global_variable_allowed() {}
+  operator T &() noexcept { return *Data; }
+
+private:
+  T *Data;
+};
+
+global_variable_allowed<int> glob;
+static global_variable_allowed<float> static_glob;
+inline global_variable_allowed<double> inline_glob;
+static const global_variable_allowed<int> static_const_glob;
+
+struct Foo {
+  static global_variable_allowed<char> d;
+};
+global_variable_allowed<char> Foo::d;
+global_variable_allowed<Foo> foo_instance;
+static global_variable_allowed<Foo> foo_static_instance;
+
+extern global_variable_allowed<int> ext;
+
+template <typename T>
+struct Bar {
+  static global_variable_allowed<T> e;
+};
+static const Bar<int> f;
+
+namespace baz {
+  global_variable_allowed<int> baz_glob;
+}
+
+// expected-error@+1{{'global_variable_allowed' attribute only applies to classes}}
+[[__sycl_detail__::global_variable_allowed]] int integer;
+
+// expected-error@+1{{'global_variable_allowed' attribute only applies to classes}}
+[[__sycl_detail__::global_variable_allowed]] int *pointer;
+
+// expected-error@+1{{'global_variable_allowed' attribute takes no arguments}}
+struct [[__sycl_detail__::global_variable_allowed(72)]] attribute_argument;
+
+
+int main() {
+  sycl::queue q;
+  q.submit([&](sycl::handler &h) {
+    h.single_task<class KernelName1>([=]() {
+      (void)glob;
+      (void)static_glob;
+      (void)inline_glob;
+      (void)static_const_glob;
+      (void)Foo::d;
+      (void)foo_instance;
+      (void)foo_static_instance;
+      (void)ext;
+      (void)f;
+    });
+  });
+}