[NFC][SYCL] Test use of sycl_kernel_entry_point with null declarations and statements. (#154583)

The `sycl_kernel_entry_point` attribute only appertains to functions.
This change adds missing tests to validate that an appropriate
diagnostic is issued for likely unintentional application of the
attribute to a null declaration or a null statement; perhaps due to an
unintended semicolon.

Author: tahonermann

clang/test/SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp

@@ -360,3 +360,16 @@ void bad38() try {} catch(...) {}
 template<typename>
 [[clang::sycl_kernel_entry_point(BADKN<39>)]]
 void bad39() try {} catch(...) {}
+
+// expected-error@+1 {{'clang::sycl_kernel_entry_point' attribute only applies to functions}}
+[[clang::sycl_kernel_entry_point(BADKN<40>)]];
+
+void bad41() {
+  // expected-error@+1 {{'clang::sycl_kernel_entry_point' attribute cannot be applied to a statement}}
+  [[clang::sycl_kernel_entry_point(BADKN<41>)]];
+}
+
+struct B42 {
+  // expected-warning@+1 {{declaration does not declare anything}}
+  [[clang::sycl_kernel_entry_point(BADKN<42>)]];
+};