C++: Avoid partial chi flow to struct/class

Flow through partial chi-instruction operands was introduced to make
definition-by-reference work, but its implementation also allowed all
other partial writes to propagate. In particular, tainting a field would
taint the whole struct, which in turn led to taint propagating across
unrelated fields of a struct.

The security test `CWE-134/semmle/argv/argvLocal.c` shows that we also
want to propagate taint from an array element to the whole array, and it
also seems right to propagate taint from a union member to the whole
union.

cpp/ql/src/semmle/code/cpp/ir/dataflow/DefaultTaintTracking.qll

@@ -189,9 +189,22 @@ private predicate instructionTaintStep(Instruction i1, Instruction i2) {
   or
   i2.(UnaryInstruction).getUnary() = i1
   or
-  i2.(ChiInstruction).getPartial() = i1 and
+  // Flow out of definition-by-reference
+  i2.(ChiInstruction).getPartial() = i1.(WriteSideEffectInstruction) and
   not i2.isResultConflated()
   or
+  // Flow from an element to an array or union that contains it.
+  i2.(ChiInstruction).getPartial() = i1 and
+  not i2.isResultConflated() and
+  exists(Type t | i2.getResultLanguageType().hasType(t, false) |
+    t instanceof Union
+    or
+    t instanceof ArrayType
+    or
+    // Buffers or unknown size
+    t instanceof UnknownType
+  )
+  or
   exists(BinaryInstruction bin |
     bin = i2 and
     predictableInstruction(i2.getAnOperand().getDef()) and