C++: Model that string functions read their buffer

[jbj]

This fixes FNs on the CWE-190/semmle/tainted/ArithmeticTainted.expected test with all open PRs applied.

It turns out that any function implementing SideEffectFunction is assumed not to read through its parameters unless hasSpecificReadSideEffect is implemented.

[jbj]

This is the diff for what's fixed:

--- expected
+++ actual
@@ -1,6 +1,9 @@
 | test3.c:15:10:15:10 | x | $@ flows to here and is used in arithmetic, potentially causing an overflow. | test3.c:11:15:11:18 | argv | User-provided value |
 | test3.c:15:14:15:14 | y | $@ flows to here and is used in arithmetic, potentially causing an overflow. | test3.c:11:15:11:18 | argv | User-provided value |
 | test3.c:15:18:15:18 | z | $@ flows to here and is used in arithmetic, potentially causing an overflow. | test3.c:11:15:11:18 | argv | User-provided value |
+| test5.cpp:17:6:17:18 | call to getTaintedInt | $@ flows to here and is used in arithmetic, potentially causing an overflow. | test5.cpp:9:7:9:9 | buf | User-provided value |
+| test5.cpp:19:6:19:6 | y | $@ flows to here and is used in arithmetic, potentially causing an overflow. | test5.cpp:9:7:9:9 | buf | User-provided value |
+| test5.cpp:19:6:19:6 | y | $@ flows to here and is used in arithmetic, potentially causing an underflow. | test5.cpp:9:7:9:9 | buf | User-provided value |
 | test.c:14:15:14:28 | maxConnections | $@ flows to here and is used in arithmetic, potentially causing an overflow. | test.c:11:29:11:32 | argv | User-provided value |
 | test.c:14:15:14:28 | maxConnections | $@ flows to here and is used in arithmetic, potentially causing an underflow. | test.c:11:29:11:32 | argv | User-provided value |
 | test.c:44:7:44:10 | len2 | $@ flows to here and is used in arithmetic, potentially causing an underflow. | test.c:41:17:41:20 | argv | User-provided value |

The problem the was lack of a read side effect node on the call to strtoul.

[rdmarsh2]

LGTM but needs test changes accepted.

[jbj]

Here's an additional improvement we're getting from this PR:

--- a/cpp/ql/test/query-tests/Security/CWE/CWE-190/semmle/tainted/IntegerOverflowTainted.expected
+++ b/cpp/ql/test/query-tests/Security/CWE/CWE-190/semmle/tainted/IntegerOverflowTainted.expected
@@ -1,3 +1,4 @@
 | test3.c:12:31:12:34 | * ... | $@ flows to here and is used in an expression which might overflow negatively. | test3.c:11:15:11:18 | argv | User-provided value |
 | test3.c:13:16:13:19 | * ... | $@ flows to here and is used in an expression which might overflow negatively. | test3.c:11:15:11:18 | argv | User-provided value |
 | test4.cpp:13:17:13:20 | access to array | $@ flows to here and is used in an expression which might overflow negatively. | test4.cpp:9:13:9:16 | argv | User-provided value |
+| test5.cpp:10:9:10:15 | call to strtoul | $@ flows to here and is used in an expression which might overflow. | test5.cpp:9:7:9:9 | buf | User-provided value |

There are also improvements in five expected files in the internal repo.