[SYCL] Fix diagnostic about non-external function/variable (#15372)

We were issuing a diagnostic when applying attributes like
[[sycl_device]] or [[intel::device_indirectly_callable]] on
functions/variables without external linkage - but the diagnostic text
assumed that this occurred only with static entities or entities within
unnamed namespaces.

The following example demonstrates a case where the previous diagnostic
text was misleading:

```
namespace {
  struct S {};
}

[[sycl_device]] void foo(S);
```

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12486,9 +12486,8 @@ def err_sycl_function_attribute_mismatch : Error<
 def err_sycl_x_y_z_arguments_must_be_one : Error<
   "all %0 attribute arguments must be '1' when the %1 attribute argument is '0'">;
 def err_sycl_attribute_internal_decl
-    : Error<"%0 attribute cannot be applied to a "
-            "static %select{function|variable}1 or %select{function|variable}1 "
-            "in an anonymous namespace">;
+    : Error<"%0 attribute cannot be applied to a %select{function|variable}1"
+            " without external linkage">;
 def err_sycl_attribute_not_device_global
     : Error<"%0 attribute can only be applied to 'device_global' variables">;
 def err_fpga_attribute_incorrect_variable

clang/test/SemaSYCL/device-indirectly-callable-attr.cpp

@@ -12,16 +12,21 @@ int N;
 void
 bar() {}
 
-[[intel::device_indirectly_callable]] // expected-error {{'device_indirectly_callable' attribute cannot be applied to a static function or function in an anonymous namespace}}
+[[intel::device_indirectly_callable]] // expected-error {{'device_indirectly_callable' attribute cannot be applied to a function without external linkage}}
 static void
 func1() {}
 
 namespace {
-[[intel::device_indirectly_callable]] // expected-error {{'device_indirectly_callable' attribute cannot be applied to a static function or function in an anonymous namespace}}
+[[intel::device_indirectly_callable]] // expected-error {{'device_indirectly_callable' attribute cannot be applied to a function without external linkage}}
 void
 func2() {}
+
+struct UnnX {};
 }
 
+[[intel::device_indirectly_callable]] // expected-error {{'device_indirectly_callable' attribute cannot be applied to a function without external linkage}}
+void func4(UnnX) {}
+
 class A {
   [[intel::device_indirectly_callable]] A() {}
 

clang/test/SemaSYCL/device_global_external.cpp

@@ -5,7 +5,7 @@
 using namespace sycl::ext::oneapi;
 
 SYCL_EXTERNAL device_global<int> glob;
-// expected-error@+1{{'sycl_device' attribute cannot be applied to a static variable or variable in an anonymous namespace}}
+// expected-error@+1{{'sycl_device' attribute cannot be applied to a variable without external linkage}}
 SYCL_EXTERNAL static device_global<float> static_glob;
 
 namespace foo {
@@ -20,10 +20,15 @@ struct RandomStruct {
 SYCL_EXTERNAL RandomStruct S;
 
 namespace {
-// expected-error@+1{{'sycl_device' attribute cannot be applied to a static variable or variable in an anonymous namespace}}
+// expected-error@+1{{'sycl_device' attribute cannot be applied to a variable without external linkage}}
 SYCL_EXTERNAL device_global<int> same_name;
+
+struct UnnX {};
 } // namespace
 
+// expected-error@+1{{'sycl_device' attribute cannot be applied to a variable without external linkage}}
+SYCL_EXTERNAL device_global<UnnX> dg_x;
+
 // expected-error@+1{{'sycl_device' attribute can only be applied to 'device_global' variables}}
 SYCL_EXTERNAL int AAA;
 

clang/test/SemaSYCL/sycl-device.cpp

@@ -12,14 +12,19 @@ int N;
 __attribute__((sycl_device(3))) // expected-error {{'sycl_device' attribute takes no arguments}}
 void bar() {}
 
-__attribute__((sycl_device)) // expected-error {{'sycl_device' attribute cannot be applied to a static function or function in an anonymous namespace}}
+__attribute__((sycl_device)) // expected-error {{'sycl_device' attribute cannot be applied to a function without external linkage}}
 static void func1() {}
 
 namespace {
-  __attribute__((sycl_device)) // expected-error {{'sycl_device' attribute cannot be applied to a static function or function in an anonymous namespace}}
+  __attribute__((sycl_device)) // expected-error {{'sycl_device' attribute cannot be applied to a function without external linkage}}
   void func2() {}
+
+  struct UnnX {};
 }
 
+__attribute__((sycl_device)) // expected-error {{'sycl_device' attribute cannot be applied to a function without external linkage}}
+  void func4(UnnX) {}
+
 class A {
   __attribute__((sycl_device))
   A() {}