C++: Add test for `__declspec` attribute on a global variable

cpp/ql/test/library-tests/attributes/var_attributes/ms_var_attributes.cpp

@@ -16,3 +16,5 @@ class AddressOfGetter {
     &field;
   }
 };
+
+__declspec("SAL_volatile") char* pBuf;

cpp/ql/test/library-tests/attributes/var_attributes/var_attributes.expected

@@ -4,6 +4,7 @@
 | ms_var_attributes.cpp:8:15:8:20 | myInt4 | ms_var_attributes.cpp:8:1:8:9 | dllexport |
 | ms_var_attributes.cpp:9:5:9:10 | myInt5 | ms_var_attributes.h:7:1:7:9 | dllexport |
 | ms_var_attributes.cpp:12:42:12:46 | field | ms_var_attributes.cpp:12:14:12:21 | property |
+| ms_var_attributes.cpp:20:34:20:37 | pBuf | ms_var_attributes.cpp:20:12:20:12 | SAL_volatile |
 | ms_var_attributes.h:5:22:5:27 | myInt3 | ms_var_attributes.h:5:1:5:9 | dllexport |
 | var_attributes.c:1:12:1:19 | weak_var | var_attributes.c:1:36:1:39 | weak |
 | var_attributes.c:2:12:2:22 | weakref_var | var_attributes.c:2:39:2:45 | weakref |