C++: pragma[nomagic] on bbStrictlyDominates

I noticed that queries using the data flow library spent significant
time in `#Dominance::bbIDominates#fbPlus`, which is the body of the
`bbStrictlyDominates` predicate. That predicate took 28 seconds to
compute on Wireshark.

The `b` in the predicate name means that magic was applied, and the
application of magic meant that it could not be evaluated with the
built-in `fastTC` HOP but became an explicit recursion instead. Applying
`pragma[nomagic]` to this predicate means that we will always get it
evaluated with `fastTC`, and that takes less than a second in my test
case.

cpp/ql/src/semmle/code/cpp/controlflow/Dominance.qll

@@ -126,6 +126,7 @@ predicate bbIPostDominates(BasicBlock pDom, BasicBlock node) = idominance(bb_exi
  * Holds if `dominator` is a strict dominator of `node` in the control-flow
  * graph of basic blocks. Being strict means that `dominator != node`.
  */
+pragma[nomagic] // magic prevents fastTC
 predicate bbStrictlyDominates(BasicBlock dominator, BasicBlock node) {
   bbIDominates+(dominator, node)
 }