C++: Add test for extensible range analysis

This commit demonstrates that the range is too wide before custom
modeling has been added to the test.

Author: jbj

cpp/ql/test/experimental/library-tests/rangeanalysis/extensibility/extensibility.c

@@ -0,0 +1,9 @@
+/// Adds its arguments (requires custom modeling in QL)
+int custom_add_function(int a, int b);
+
+int test_extensibility_add(int x) {
+  if (x >= -10 && x <= 10) {
+    int result = custom_add_function(x, 100);
+    return result; // should be 90 .. 110
+  }
+}

cpp/ql/test/experimental/library-tests/rangeanalysis/extensibility/extensibility.expected

@@ -0,0 +1,4 @@
+| extensibility.c:5:7:5:7 | x | -2.147483648E9 | 2.147483647E9 |
+| extensibility.c:5:19:5:19 | x | -10.0 | 2.147483647E9 |
+| extensibility.c:6:38:6:38 | x | -10.0 | 10.0 |
+| extensibility.c:7:12:7:17 | result | -2.147483648E9 | 2.147483647E9 |

cpp/ql/test/experimental/library-tests/rangeanalysis/extensibility/extensibility.ql

@@ -0,0 +1,8 @@
+import semmle.code.cpp.rangeanalysis.SimpleRangeAnalysis
+import experimental.semmle.code.cpp.models.interfaces.SimpleRangeAnalysisExpr
+
+from VariableAccess expr, float lower, float upper
+where
+  lower = lowerBound(expr) and
+  upper = upperBound(expr)
+select expr, lower, upper