C++: Speed up SuspiciousAddWithSizeof select

This `select` clause had become very slow after we started caching
`ElementBase::toString` because the query used string concatenation to
produce alert messages, and those string concatenations were done very
early in the pipeline, producing lots of strings that would be discarded
moments later.

By using `$@` to interpolate elements into strings, the concatenation is
done outside of QL.

Testing on a Chromium snapshot, this commit takes us from

    #select#ff ................ 6m2s

to

    #select#cpe#134#fff ....... 15.2s

Author: jbj

cpp/ql/src/Security/CWE/CWE-468/SuspiciousAddWithSizeof.ql

@@ -28,5 +28,5 @@ where
   // is probably a mistake.
   addWithSizeof(e, sizeofExpr, _) and not isCharSzPtrExpr(e)
 select sizeofExpr,
-  "Suspicious sizeof offset in a pointer arithmetic expression. " + "The type of the pointer is " +
-    e.getFullyConverted().getType().toString() + "."
+  "Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@.",
+  e.getFullyConverted().getType() as t, t.toString()

cpp/ql/test/query-tests/Security/CWE/CWE-468/semmle/SuspiciousAddWithSizeof/SuspiciousAddWithSizeof.expected

@@ -1,7 +1,7 @@
-| test.cpp:6:30:6:40 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is int *. |
-| test.cpp:14:30:14:40 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is int *. |
-| test.cpp:22:25:22:35 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is int *. |
-| test.cpp:30:25:30:35 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is int *. |
-| test.cpp:38:30:38:40 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is int *. |
-| test.cpp:61:27:61:37 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is int *. |
-| test.cpp:89:43:89:55 | sizeof(MyABC) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is myInt *const. |
+| test.cpp:6:30:6:40 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | int * | int * |
+| test.cpp:14:30:14:40 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | int * | int * |
+| test.cpp:22:25:22:35 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | int * | int * |
+| test.cpp:30:25:30:35 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | int * | int * |
+| test.cpp:38:30:38:40 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | int * | int * |
+| test.cpp:61:27:61:37 | sizeof(int) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | int * | int * |
+| test.cpp:89:43:89:55 | sizeof(MyABC) | Suspicious sizeof offset in a pointer arithmetic expression. The type of the pointer is $@. | file://:0:0:0:0 | myInt *const | myInt *const |